Migrate to GTK 4

[nonetrix]

GTK 4 would future proof the project and add better support for CSS and brings GPU acceleration might or might not be worth it so I made this a discussion instead

[davfsa]

Could be something worth looking at.

I am not sure how easy it would be to migrate, nor how feature complete Astal is in regards to GTK 4, but worth looking at IMO :)