In future we will need to use window clones to better support multiple
monitors. To avoid having to hide every window, show wallpapers behind
the workspace transition: one per monitor.
Put the wallpaper into a separate class right away, later it will be
useful to make the animation per-monitor.
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1326
Simplify the code a bit. The workspace group is relatively self-contained,
so split it from the general animation. Reimplement _syncStacking().
This will help a lot later, with workspace strip and multi-monitor support.
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1326
