threadshare: consider separating `Task` from `PadSrc`
This would allow defining a PadSrc without the need for a Context, which is currently mandatory when calling prepare. Elements which require a Task could instantiate and manage it directly. This would probably be best done via the matching PadSrcHandler implementation.
Additionally, now that the unprepare methods are no longer async, we could make sure to call them as part of the Drop implementations for PadSrc, PadSink and Task.