Add GNOME Mutter backend plugin
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.