Draft(?): port to gtk4

For the gtk4 port, it is meant to be the same as the before, just with GTK4, changes only what is sufficiently required.

From testing, everything works, however it is very hard to run on host where settings have effect due to the dependencies, and because gnome-desktop v4.0 breaks GNOME Session.

However here is a demo video (note that some settings aren't available as running in toolbox, however I did check they work once): gnome-tweaks-gtk4-demo

Some possible issues:

• Libadwaita and the theme tweak. Adw.init() currently force-enables the Adwaita theme and makes the custom GTK4 theme not followed. This is quite bad as the GTK Theme combobox appears to not do anything, even if it does work. Hopefully libadwaita reconsiders this. (Works without Adw.init() but that has its own disadvantages.
• The Compose dialog looks a bit weird on X11, however it looks correctly on Wayland.

Adding this link for "Mentioned in merge request !xx" to appear: Initiatives#26

Edit: also about the relation of this to !76 (merged). I have also changed the leaflet code here a bit, so this also fixes the issue described in that merge request (so that one isn't really needed with this one) (and I also saw that reddit post too)