Add GTK4 GUI

[fedorenchik]

Is your feature request about something that is currently impossible or hard to do? Please describe the problem.
GTK4 supports hardware rendering, would be nice to have support for it.

Describe the solution you'd like
Support GTK4 GUI.

Describe alternatives you've considered
GTK3, but no hardware acceleration.

Additional context
As a bonus, can an also remove support for GTK2, since GTK2 is deprecated.

[k-takata]

Official document for migrating 3 to 4:
https://developer.gnome.org/gtk4/stable/gtk-migrating-3-to-4.html
PR is welcome.

[fictitiousexistence]

Got 404 with that link. Looks like this is the new page:

https://docs.gtk.org/gtk4/migrating-3to4.html

[dvogel]

I'm experimenting with dropping GTK2 support and adding GTK4 support. Reading through gui_gtk_f.c, it sure seems like this is a custom implementation of GtkFixed. However, that widget has existed since GTK2 and it was not used. Does anyone know why not?

[dvogel]

I have a draft PR open for discussion on how to best support the mouseshape option with GTK3 and GTK4 CSS mouse cursor names. Please do chime in over there if you depend on this feature (I don't).