Ubuntu specific installer used for InputMethod installer

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Describe the bug
I'm on NixOS, which isn't going to have apt at all, and it appears to be programmed in a way that assumes an apt client exists. At the very least it could bail out with a dialog or only add the `Install Unlisted Engines` button if an apt client exists.
I currently just get this error in the console
```
UbuntuInstaller.vala:75: Could not queue downloads: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.debian.apt was not provided by any .service files
```

### To Reproduce
1. Be on a platform without apt
2. Click on Input method view
3. Click the :heavy_plus_sign:
4. Click `Install Unlisted Engines` and select an engine
5. :boom: what happened? (hint: it appears like nothing)

### Expected behavior
It's possible to use packagekitd this without being specific to Ubuntu.
If it isn't present I shouldn't even be able to access this in the interface.
(On a side note, nix will have support for packagekitd in Nix 3.0)

## Platform Information
- OS: NixOS
- OS Version: 20.09-pre-git
- [ ] I'm using the latest version from git that I've manually compiled
- [x] I'm using the latest released stable version