Preparations for GTK4

A grabbag of pre-gtk4 changes + some code cleanups

The changes in ui files are pretty invasive, so marking this as WIP until @1PunMan's MRs are in.

There will be more changes once GTK+ 3.24 is in runtime, mostly related to event handling.

There are also some actual UI changes:

• Close button on loading notification has a proper symbolic icon now;
• Colors in gamepad configuration can be slightly different on some themes, including dark (but not light) variant of Adwaita.