Not only this way we get the right Adwaita cursor as the default
(instead of shipping our own in png format), but we also add
support for all MetaCursors as root cursor (which most important
should allow us to have I-beams in shell entries)
https://bugzilla.gnome.org/show_bug.cgi?id=707573
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
Under X, we need to use XFixes to watch the cursor changing, while
on wayland, we're in charge of setting and painting the cursor.
MetaCursorTracker provides the abstraction layer for gnome-shell,
which can thus drop ShellXFixesCursor. In the future, it may grow
the ability to watch for pointer position too, especially if
CursorEvents are added to the next version of XInput2, and thus
it would also replace the PointerWatcher we use for gnome-shell's
magnifier.
https://bugzilla.gnome.org/show_bug.cgi?id=705911
