Magnifier: fix a warning when calling setActive() with the same value

We must not call enable/disable_unredirect if we didn't change the
active value, otherwise mutter gets confused and emits warnings.

https://bugzilla.gnome.org/show_bug.cgi?id=724293
Magnifier: demote exceptions reading focus/caret position

It's possible that the DBus call goes in timeout (which is bad,
but unavoidable, given that atspi is synchronous) or fails
because the component has been removed (race condition). Those
errors are not dependent on gnome-shell, but on faulty applications
(mostly).
If they happen, log a debug message and continue.

https://bugzilla.gnome.org/show_bug.cgi?id=724305
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
Magnifier: use the system noise texture for the dead area

The design says that the noise texture is the implicit bottom
layer, and so it's appropriate to use it togheter with the default
color to cover the dead area outside the screen which becomes
visibile when scrolling.

https://bugzilla.gnome.org/show_bug.cgi?id=724305
Magnifier: clip the UI group clone to the allocation

Without clipping, we show parts of the message tray and workspace
thumbnails that are normally clipped by the screen.

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