MetaCursorTracker: extend with query pointer abilities

We need an abstraction in gnome-shell for XQueryPointer, and
MetaCursorTracker seems a good place for it.

https://bugzilla.gnome.org/show_bug.cgi?id=707474
MetaCursorTracker: add methods for setting the cursor visibility

clutter_stage_show_cursor()/hide_cursor() only works in the X11
backend (where someone else is in charge of showing the cursor),
and even then, it has confusing effects when running nested wayland,
so an abstraction layer is needed.

https://bugzilla.gnome.org/show_bug.cgi?id=707474