0f99d46f87
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. Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1326>