Fix switching of active window.

When getting a surface leave/surface enter pair, the enter event was lost.
It could be seen when opening a popup menu. This patch uses a wl_callback
to activate the right window.

Change-Id: Ibd35fcaa0d43f4723dd7aeff71264d6145411a08
Reviewed-by: Andy Nichols <andy.nichols@digia.com>