Use a system-wide Tracker domain instead of trackerappdomain
Since the app is now only officially supported inside a Flatpak bundle or installed into /usr, we no longer need some of the functionality from trackerappdomain. We could instead install uk.me.afuera.Tagcloud.rule into /app/share/tracker/domains and have our daemons run on the real session bus.
This would allow simplifying trackerappdomain code significantly, hopefully eventually deleting it altogether.