GTK4 port

[ErikReider]

Fixes: #134 and #268

Still some stuff to finish before it's ready.

Todo:

• [ ] Fix random segfaults. (Open Gtk4-layer-shell issue here)
• [ ] Fully transparent BlankWindows aren't visible. (Maybe adjust input/opaque region manually?)
• [ ] The Control Centers surface enter_monitor signal is only called once to get the current monitor. Listn to another signal instead?
• [x] Make sure that all widgets work/look the same/similar to the prior version
• [ ] Fix MPRIS carousel not being scrolled when window is hidden (should be fixed by the next libadwaita version)
• [ ] Fix large notification bodies height not being compensated for in the ListView (https://gitlab.gnome.org/GNOME/gtk/-/issues/5885)