Switch to Gtk.Application

[Cimbali]

The Gtk.Application framework seems to be meant for simplifying code, which using glade files and a Gtk.Builder did quite a bit already.

So if the change is worth it in code clarity etc, then switch to that approach.

I'm afraid that for example we'd have to write out all the Gio.Action by hand in the code, as in the demo on readthedocs. Or that having two windows content/presenter with only one with a menu bar could become messy.

[Cimbali]

Won't be doing this, as it does not look it will make the code any more readable.