Add a new 'origin' field to queue entries. This records the origin of

Add a new 'origin' field to queue entries.  This records the origin of
the track independently of its current state (the 'state' field mixes
up the two concepts in an unhelpful way).  A few things use this but
not yet all.

Old queues will produce confusion if you upgrade to this version.
Nothing's yet been done to fix this; there'll have to be a version
field.

Fixed a few bits of template documentation too.