delete threaded workers on a thread
this prevents potential deadlocks between the worker thread and the main thread in case the former relies on the latter to do work. specifically qdbus seems unhappy if we wait in the main thread. in point of fact we shouldn't be waiting in the main thread anyway since the result of the thread is immaterial
(wait is also called inside the workerthread destructor, I'm leaving it there as a safe guard against deleting still-running threads)