MBS-5397: edit medium tracklist display

Makes the edit display look like this:

![edit-medium](https://f.cloud.github.com/assets/106004/91331/e5655288-659a-11e2-994b-efdcf447f155.png)

Instead of like those linked from http://tickets.musicbrainz.org/browse/MBS-5397

This works a lot better with track insertions and deletions, since the diff computation ignores track numbers/positions. It also always displays the whole tracklist, and displays the old/new side-by-side for better comparison.

Testing: manual. When nothing is changed, tracklists are still not displayed. If only numbers change, it *will* be displayed.