xwm: Fix memory leak

xwm: Fix memory leak

A memory leak introduced by 6b58ea8c led to me finding a bigger leak,
which is xwm was calling frame_create() without calling frame_destroy().
This meant that the associated icon_surface was not being destroyed,
leaving the destroy handler for it broken. Here we fix this by calling
frame_destroy() when the window is destroyed and free the reply in
the icon_surface destroy handler.
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>