Migrate to GTK4

My feelings with GTK are very good, although the GTK4 branch has yet to reach feature parity with the Qt version (specifically file saving). I'm almost sure I'm going to merge this even if I don't get it to look good on Windows. I might even go back to using libadwaita (😮) since I'm only using GNOME anyways but I won't if I find a way to make GTK4 look more like libadwaita on GNOME.