Refactoring for database integration:
1. Track.[prev, next] attributes now return the id values instead of Track instances.
2. Track.[previousTrack, nextTrack] have been added to return Track instances in place of the prev and next.
3. Track.[prevTrackNode, nextTrackNode] have been added return the node name of the previous or next track if applicable (i.e. Maya scene is active).
4. TrackPropEnum.NEXT has been removed and now the next attribute is computed in both the Maya and database cases from prev relationships.