Port from GTK 3 to GTK 4

[Conan-Kudo]

QGnomePlatform currently uses GTK 3 for the GTK/GNOME-side bindings of things. GNOME is increasingly using GTK 4, so QGnomePlatform apps look a bit odd as more of GNOME switches to GTK 4.

Consider porting the GTK 3 code to GTK 4 in QGnomePlatform.

[grulja]

I'm not sure it makes sense to port it to GTK 4, I would rather drop GTK completely and rely only on xdg-desktop-portal which will give you both, depending on the portal backend implementation.

[grulja]

Ok, I'm not giving up on GTK4. I started https://github.com/FedoraQt/QGnomePlatform/tree/wip/gtk4, but I will have to ask someone more experienced to help me with the port as lots of the API we use is gone.