Decide if OperationWorker progress should be an event or a bare delegate

[joflashstudios]

The need to transfer instructions back (hey, stop!) might merit using a delegate. Though, the event should always run on the same thread, so setting a property would theoretically work, if done before control is passed out of the event handler.

[joflashstudios]

Should be an event. Sender can be used to stop, etc.