Plasmashell notifications accept focus unless no_focus directive is used

Per [discussion here](https://github.com/i3/i3/pull/4067), I'm creating a separate issue. There is an alternative way to reproduce it by using picom/compton with transparency, and switching to another workspace *(which makes notification to get focus)*.

A workaround for the problem described is to add in config this line:

```
no_focus [window_type="notification"]
```

## I'm submitting a…

[x] Bug

[ ] Feature Request

[ ] Documentation Request

[ ] Other (Please describe in detail)

## Current Behavior

A notification popup makes keyboard layout get changed again

## Expected Behavior

Keyboard layout stays the same

## Reproduction Instructions

1. Start KDE Plasma
2. Open `systemsettings5 → Input Devices → Keyboard → Layouts`
1. Make sure you have more than one language layout
2. Set "Switching policy" checkbox to "Window"
3. Click `Apply`
3. Create a `/usr/share/xsessions/plasma-i3.desktop` file with the following:
```
[Desktop Entry]
Type=XSession
Exec=env KDEWM=/usr/bin/i3 /usr/bin/startplasma-x11
DesktopNames=KDE
Name=Plasma with i3
Comment=Plasma with i3
```
4. Relogin into session named "Plasma with i3" and wait for it to finish loading *(a plasma panel should appear at the bottom)*
5. Write in terminal `notify-send 'Hello world'`
6. Switch keyboard layout to non-default one
7. Press enter

## Environment

Output of `i3 --moreversion 2>&-`:

Binary i3 version:  4.18.1 (2020-04-22) © 2009 Michael Stapelberg and contributors

(Getting version from running i3, press ctrl-c to abort…)

Config file

https://pastebin.com/raw/kfhcrzua

- Linux Distribution & Version: Archlinux

- Are you using a compositor: picom, but it is reproducible with and without it.