Magnifier: don't listen for focus/tracker events if the magnifier is not active

Magnifier: don't listen for focus/tracker events if the magnifier is not active

In addition to checking the current settings, check also if the
zoom region is active before registering the event listener.
This way, we avoid DBus traffic for events we're not interested in.

Also, make FocusCaretTracker resilient to multiple register/deregister
calls (which can now happen).

https://bugzilla.gnome.org/show_bug.cgi?id=724305