Add GNOME Mutter backend plugin

[romangg]

Mutter output configuration is controlled via D-Bus: xml-description

We should be able to just add another backend plugin to our Wayland backend for that. As with the other backend plugins we just check in the loop if there is a (this time D-Bus) interface announced and if yes the plugin is selected.

One interesting difference might be xdg-portal support. But we can also first add it without that.

[romangg]

In GitLab by @Kodehawa on Dec 4, 2020, 19:42

This is a little tricky. On X we should be able to do this properly as RandR backends shouldn't conflict (though it definitely will, at login), but on Wayland (and on X, in a way), we'd be fighting the ever-present gnome-settings-daemon, which can't be disabled. Change gnome for cinnamon or budgie for the other desktops.
As far as I know, there's no modularity to it.

If we wanna achieve this without hackery, I think we should get in contact with the GNOME guys to see if there's a way to delegate display configuration to something that's not gnome-settings-daemon.