Avoid deadlock during window hiding.

Because we do event processing in polishAndSync, hide() or
handleObscurity() might have been called by the time we
continue execution. We already handled that windowDestroyed()
was called.

Change-Id: If45bcf3fa18cef1a60ca31bc26d3830045f581d9
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Merge pull request #31 from sletta/mer-stable

[qtdeclarative] Avoid deadlock during window hiding.