index: Make deep index sorting behavior slightly nicer.
index: Make deep index sorting behavior slightly nicer.

Since Song objects default to a track number of 0, song collections
that lack track tags aren't sorted correctly in the deep index.

This applies a heuristic - if any of the track numbers are greater than 0,
sort via track metadata. Otherwise, sort by pathname on the assumption that
files are named like "01 song name.mp3".

woodruffw/muzak,muzak-project/muzak