WIP: overview: Treat desktop extensions the same as desktop windows
We don't have desktop windows anymore since those came from
nautilus-desktop. But we can reuse the same function to return a
desktop-style actor if one has been added by extension, like
https://gitlab.gnome.org/World/ShellExtensions/desktop-icons
This allows the desktop icons to fade out and in with the overview, just like Nautilus used to, using the same logic written for Nautilus.
Edited by Daniel van Vugt