Add Gtk.Application subclass for minor cleanup

Ideally we'd like Boxes.App do inherit from Gtk.Applicaton, but we can't
as it needs to inherit Boxes.UI. Instead we create a subclass as
a proxy object that calls into Boxes.App. This makes the "app"
callbacks real methods rather than signal connections which look
slightly nicer, and allows later overriding of Gtk.Application vfuncs.

https://bugzilla.gnome.org/show_bug.cgi?id=692306