MonitorConstraint: Use actor property notify to monitor changes

MonitorConstraint was overriding the set_actor vfunc to be notified of the
constraint actor changes, however ClutterActorMeta calls this virtual function
during its disposition and this is a problem if the disposition is triggered by
the JS Context shutdown, because at this point the JS code is not accessible
anymore, causing unexpected behaviors, other than a critical gjs error.

So, instead of using the virtual function, monitor the actor changes by using
the property notification.

Depends on GNOME/mutter!682

GNOME/gnome-shell!648
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