Port to GTK4 [WIP] [possible?]

[phuhl]

Most of the port is done already, but changes in GTK4 have blocked some roads for us. The single most important feature that has been removed is the ability to position windows. Quite a bummer.

Over here might be a starting point to build a little bit of glue code to regain this ability with GTK4.

Other issues:

• no more click events on Box-elements afaik, this was used in multiple places (closing of the center on click, closing of notis on click)

Why would we do a port to GTK4?

• Better CSS styling for pictures
• Hopefully, now one can set the width of windows explicitly (read, it would be a feature, needs confirmation)
• Hopefully, better alignment of images in notifications possible

[phuhl]

Yea, talking about eww.
My wayland """solution""" for now is to not support the window-positioning side of things in wayland, but to instead just do nothing there. Then, I'll try to write some documentation about how to configure window position via rules in some of the more popular wayland compositors - doing that should be possible in most compositors.
As the rendering still works, wayland is still """supported""", just missing some key things that will need to be replicated on the compositor side.

The relevant code I wrote to replace the removed GTK calls is pretty much all in https://github.com/elkowar/eww/blob/gtk4/src/display_backend/x11.rs, being used in https://github.com/elkowar/eww/blob/gtk4/src/app.rs
Still imperfect, but I've managed to replicate the old gtk functionality moooooostly, with a tiny bit of jankyness in some rare edgecases.