Remove automated queueing of tracks and rework the queue for other purposes.

The queue is now not actually used, but can have Track objects enqueued onto
it, at which point it will be used as soon as possible; when the queue is
empty, which (in current code) is always the case, a track is chosen by magic.
Merge pull request #103 from Rosuav/queue_change

Remove automated queueing of tracks and rework the queue for other purposes.