shell/app-system: Retrack windows if necessary

shell/app-system: Retrack windows if necessary

We currently don't update running apps when the set of installed
apps changes. However we should, as the change could mean that

 - the app got uninstalled, and the running app should become
   window-backed

 - a newly installed app got launched before the app cache was
   updated, and we replace the window-backed fallback with the
   proper app

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5664

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2480>