Emit ScreenChanged events on X11

We do not currently emit screenchanged events when a window is moved
to another screen on X11.

This patch emits the event when a window no longer intersects with
its current screen, and switches to the first intersecting screen.

Change-Id: Ie40d6eb67b85bd961eeb348bc43e4f308ee22dba
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Uli Schlachter <psychon@znc.in>