This removes the responsibility of tracking these from the backend to
the base object. The backends are instead responsible for calling the
function to update the values.
For the native backend, it's important that this happens on the correct
thread, so each time either of these states may change, post a idle
callback on the main thread that sets the, at the time of queuing said
callback, up to date state. This means that things on the main thread
will always be able to get a "new enough but not too new" state when
listening on the 'notify::' signals and getting the property value
after.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1739>
We thus far have similar objects/code internal to backends. Expose the
minimum API necessary to cater for gnome-shell as a generic object.
So far only the X11 backend has an actual GObject for it, and was made
to be a subclass right away.
