Break libgnome-desktop into three separate libraries:
- gnome-desktop-4, containing the base data types and API, under the GnomeDesktop namespace
- gnome-rr-4, containing the
gnome_rr_*API, under the GnomeRR namespace - gnome-bg-4, containing the
gnome_bg_*API, under the GnomeBG namespace
We keep a full gnome-desktop-3.0 for backward compatibility of components that still use GTK3, while the gnome-rr and gnome-bg libraries are ported to GTK4 until we can either split them off, or drop them.