Start download in advance when queueing

[ejv2]

There is no point in queuing an episode of something which hasn't been downloaded, only to wait until it wants to be played to download it. If the user has enqueued something, he clearly wants to listen to it in the near future, so I reckon we should probably put any episodes in the download queue if they have been added to the play queue automatically.

Could implement this by checking at the time of enqueuing, or could possibly also do a check of the queue at each track change. It would probably be smoother to do it at the time of queue mutation, as that avoids the possibility of unnecessary waiting altogether.

All existing behaviour involving the play and/or download queues will be unchanged.