window: Fix delayed mouse mode

Using clutter_actor_has_pointer() to test whether the pointer is
on the window makes for clean and nice-looking code, but does not
work in practice - ClutterActor:has-pointer is not recursive, so
we miss when the pointer is on the associated surface actor rather
than the actor itself.
Instead, check whether the window actor contains the core pointer's
pointer actor, which actually works.

https://bugzilla.gnome.org/show_bug.cgi?id=730541
window: Fix delayed mouse mode on X

On X, basing the check whether the pointer is on the window on
Clutter events does not work, as the relevant events are handled
by GDK instead.
So add an X-specific window_has_pointer() implementation to also
fix mouse mode when running as X compositor.

https://bugzilla.gnome.org/show_bug.cgi?id=730541