window: free close dialog before unmanaging window from compositor
When an application stops responding, the shell darkens its windows.
If a window from a not-responding application gets unmanaged then the shell will currently throw an exception trying to retrieve the now-dissociated window actor.
That leads to a "stuck window" ghost on screen and a traceback in the log.
This commit addresses the problem by making sure the effect is cleaned up before the actor is disocciated from its window.