Winrt: QThread: take time into account in QThread::wait

Instead of just joining the thread using WaitForSingleObjectEx
(and join afterwards) allows us, to take the actual time to
wait into account and a proper return value can be given.

Change-Id: I52b8612c0c91483d43873999efd0ed4421e95e10
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>