build: Switch to gnome-desktop-4
libmutter does no longer pull in gtk3 so we don't need to be stuck with gnome-desktop-3.0 anymore. Lets move to the gtk-free gnome-desktop-4.
libmutter does no longer pull in gtk3 so we don't need to be stuck with gnome-desktop-3.0 anymore. Lets move to the gtk-free gnome-desktop-4.