Replace ShellXFixesCursor with MetaCursorTracker

Mutter now includes an object with the same purpose and functionality
as ShellXFixesCursor, so we can replace our XFixes code with it
and work under wayland too.

https://bugzilla.gnome.org/show_bug.cgi?id=705911
ShellRecorder: update to use MetaCursorTracker

Replace more direct XFixes usage with a the appropriate abstraction
API from mutter, which is guaranteed to work in wayland too.

It doesn't yet replace pointer position tracking, although probably
it should.
Also, because now we're using Mutter API, we lose the standalone
test case.

https://bugzilla.gnome.org/show_bug.cgi?id=705911
ShellScreenshot: replace XFixes usage with MetaCursorTracker

More X11 code going away from gnome-shell, and more wayland
compatibility.

https://bugzilla.gnome.org/show_bug.cgi?id=705911