add data_source, data_url to TrackInfo objects

The Beatport plugin now uses this to provide singleton matches (and their
appropriate penalty).
show penalties and disambig for tracks

This makes the UI for track match display match that for album display.
show penalties/disambig in track listing

This is the second step in bringing parity between the album and item
displays. Fortunately, we get to reuse most of the code.
show data URL for track matches

That should be the last one.