Complete rewrite of the core of Music
Use persistent list models throughout Music to hold the state of a users music collected through Grilo and Tracker. The extent of the rework did not allow for a gradual approach, so this commit marks a hard break from the old approach. It is not yet on-par with the old codebase. This rework is partially based on the work of Jean Felder <firstname.lastname@example.org>. Related: #299.