WIP: Remove gnome-bg
It's now moved to libgnome-bg.
Only 2 users in GNOME:
- g-c-c
- xdg-desktop-portal-gtk
And a couple in GNOME Flashback. It doesn't really make sense to have as a widely available library, as we try and trim libgnome-desktop away.
See https://github.com/flatpak/xdg-desktop-portal-gtk/issues/250