Playlist: add sorterTracks function in header
Playlist/PlaylistView: fix all drag reordering of playlists/tracks
- No longer use Playlist#insertTrack to do things so this can be
  simplified just to deal with single insertion using gap method
- Fix drag reordering in PlaylistView#acceptDrop. This now does the
  following procedure:
    - Calculate insertion point by counting how many to-move items are
      above the proposed row and subtracting this from the row
    - Renumber the other items around the hole
    - Renumber the to-move items
- Closes #31