Gtk4 and Wayland

[Thiago Milczarek Sayão]

Hi,

I did some investigation on gtk4 and wayland.

After some research I ended up with the conclusion that the best way is to
do a separate gtk4 backend, that would support X11 and Wayland.

This would be a good start:
https://github.com/openjdk/jfx/pull/77/files

Why?

Gtk4 moves the decoration to the client side, which is GREAT since knowing
the window size with decoration was a real pain.

We probably won't want to do all the decoration work, Gtk does that, but on
GtkWindow level, not GdkSurface (which replaces GdkWindow).

Thus the move to use "more Gtk" (hence "less Gdk") which is exactly what
the PR does. It also removes Applet code

This is also a good starting point:
https://gnome.pages.gitlab.gnome.org/gtk/gtk4/migrating-3to4.html

--Thiago.