Refactor Magnifier.ZoomRegion to avoid permanent Clutter.Clone

This basic point of this change is to avoid always creating a
hidden Clutter.Clone actor for the default present-but-not-active
zoom region. The position of the viewport and region of interest
are now stored in member variables, and the actors are only created
and updated when the region is active.

Other significant changes:

 * Unused public functions are removed or made private
 * The mouse tracking position is immediately  updated when options
   like the zoom are changed, not just on the next mouse motion.
 * ZoomRegion.setROI() now updates the zoom, not just the position;
   a FIXME is added to the D-Bus interface for a place where the
   D-Bus interface contains duplicate possibly conflicting information
 * Lens-mode is now only effectively off when the magnifier is
   fullscreen, instead of actually modifying the member variable;
   this makes things work properly when changing out of full-screen
   mode.
 * When the clamping to screen edges is turned on, we now immediately
   clamp.
 * The handling of setting the position to fullscreen as compared
   to just setting the viewport to fullscreen is untangled.

https://bugzilla.gnome.org/show_bug.cgi?id=633582