Port to GTK 3, GtkSourceView 4 and Vte-2.91

Debian will be removing GtkSourceView 2 shortly and probably GTK 2 within the next release cycle, which would put a useful program like Genius at risk of being removed from the distribution. So here's an attempt to port to GTK 3 and modern versions of Vte/GtkSourceView. There could be regressions here and there; I tried to test thorougly but it's quite possible that I've missed something.

I decided to use Amtk because that's the easiest way to migrate from GtkUIManager while still preserving nearly the same look & feel (which I like). If that's undesirable for some reason just let me know and I'll rework it to use plain GTK.