ShellGlobal: use MetaCursorTracker to query the pointer position

Gdk uses Xwayland, so it only sees the events we forward to X11
clients. Instead, we can use the abstraction API provided by
mutter and get the right value automatically.
Also, we need to use MetaCursorTracker to handle the cursor
visibility too.

https://bugzilla.gnome.org/show_bug.cgi?id=707467