WIP: GTK4 port

This is same as !81 (closed), but with a branch in this repo instead of my fork.

It's still way too early to merge it, but here it is.

This MR depends on GNOME/retro-gtk!23 (merged), !78 (merged) and !80 (closed). (currently just includes them so that it builds)

Remaining problems to fix:

• It crashes after exiting any libretro game, see GNOME/gtk#1423 (closed)
• Some runtime warnings are printed in keyboard page in preferences;
• Search-as-you-type is using a deprecated-but-not-deprecated function that is somehow isn't removed yet; :) Doesn't work anymore
• Developers and platforms view are initially empty for some reason. Doesn't happen anymore, probably one of Yetizone's changes fixed it
• Preferences window doesn't open the first time. Was a hidden bug, fixed in https://gitlab.gnome.org/GNOME/gnome-games/commit/b8ce6d86258410be8b51542ca5e8345576ac959d
• Controller tester/mapper page is not horizontally expanded Doesn't happen anymore with !159 (merged)
• Gamepad view uses integer scaling for some reason
• Since libhandy isn't ported to GTK4, I've stubbed it out for now in the first commit