Improved performance, we now do not go to the X server in order to get the list of...

Improved performance, we now do not go to the X server in order to get the list of window children. Apparently the effect is the same, we have to be careful and check if this breaks something in any corner case.

* src/hildon-pannable-area.c:
	(get_ordered_children): Removed this function.
	(hildon_pannable_area_get_topmost): Replaced the ad-hoc search of
	the children windows function with gdk_window_get_children.