[gui] need to limit the number of instances listed

**Describe the bug**

The indicator lists all the VMs I have used. I want to shell into one. I find it in the list and hit the triangle. It opens up a small section which I need to scroll to find the "Shell" option.

**To Reproduce**
1. Have 27 VMs created with multipass
1. Set screen to 1080p on GNOME Shell
1. Try to get to the shell option for the bottom VM

You get a tiny tiny scroll bar and it's very hard to use.


**Expected behavior**
A menu to appear, from which I could choose "Shell"

**Additional info**
- OS: Ubuntu 20.04
- `multipass version`
```
multipass 1.1.0
multipassd 1.1.0
```

- `multipass info --all`
```
Name: primary
State: Running
IPv4: 10.186.93.147
Release: Ubuntu 18.04.4 LTS
Image hash: fe3030939822 (Ubuntu 18.04 LTS)
Load: 0.00 0.00 0.00
Disk usage: 1.1G out of 4.7G
Memory usage: 77.0M out of 985.1M
Mounts: /home/alan => Home
UID map: 1000:default
GID map: 1000:default

Name: snapcraft-broot
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/broot => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-codium
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/codium => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-dapper-drake
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/dapper-drake => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-dosbox-x
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/dosbox-x => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-ephemeral
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/ephemeral-snap => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-feeds
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/feeds => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-imager
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/imagewriter => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-jellyfin
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/jellyfin => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-lucid-lynx
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/lucid-lynx => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-mangohud
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/mangohud => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-monodevelop
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/monodevelop => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-netris
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/netris => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-overviewer
State: Stopped
IPv4: --
Release: --
Image hash: 5e3c64fd604d (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/overviewer => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-precise-pangolin
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/precise-pangolin => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-pyplane
State: Stopped
IPv4: --
Release: --
Image hash: cb460e7b363e (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/PyPLANE => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-qb64
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/personal/qb64 => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-rpi-imager
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/rpi-imager => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-session-desktop
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/session-desktop => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-simplenote
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/simplenote => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-snapcraft
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/snapcraft => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-sosumi
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/sosumi-snap => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-srain
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/srain => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-warp
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/warp => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-warty-warthog
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/warty-warthog => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-whatip
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/whatip => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-xenial-xerus
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/xenial-xerus => /root/project
UID map: 1000:0
GID map: 1000:0

Name: snapcraft-zzt
State: Stopped
IPv4: --
Release: --
Image hash: 2ca8b726be5b (Ubuntu Snapcraft builder for Core 18)
Load: --
Disk usage: --
Memory usage: --
Mounts: /home/alan/Source/personal/zzt-snap => /root/project
UID map: 1000:0
GID map: 1000:0

```
**Additional context**

(maybe some of these old VMs that I'm using with snapcraft over the last few weeks should be cleaned up if not used)

Start by reproducing the GUI with many instances on a 1080p Ubuntu desktop, following the listed steps and using the screenshot as a reference. Trace the instance-list and Shell-action entry points, then confirm that the Shell option remains readily accessible without an unusably small scrollbar when many VMs are present.

Written by the indexing model from the issue text.