Use app info launch

The idea here is to use the GLib API, so that we can stop caring about moving applications into scopes ourselves. Also, the glib code is (and can be) smarter about how it does this.

Note that this will trigger crashes without mutter!1392 (merged).

I think this should be safe to merge, but it is in no way urgent (and, there is the possibility that some quoting stuff in the Alt+F2 dialog will now behave in different ways).