Add an option to sort Grooveshark playlist alphabetically. Fixes issue #3480.
Did it the lazy way for now, but it would have better to refactor Grooveshark code, to add a proper sort model, etc.
Grooveshark: don't crash when receiving the reply for a canceled request.
Mark all tasks as finished when removing all items, to prevent never ending task in case we disconnect or refresh before all replies have been received.