This is mainly just a start on removing
Gtk.Container in pygobject for GTK4.
As I see it there are 2 approaches:
Containercreate 2 classes, one with the inheritance and one without and a big
if/else statement around it.
Container empty class in the case of GTK4 and just inherit that.
This MR went with option 2 to start out, but I'm fine doing either of them.