app-system: Filter out stale apps from cache

Since rebasing our AppSystem on GLib's facilities, we only ever
append to the id-to-app cache. So if an application is uninstalled,
shell_app_system_lookup_app() will still happily return it if it
was cached previously. For instance if a favorite app is uninstalled,
it keeps lurking in the dash until a restart.
To fix, filter out removed apps from the cache when handling
GAppInfoMonitor::installed-changed.

https://bugzilla.gnome.org/show_bug.cgi?id=726414