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 <jfelder@src.gnome.org>.

Related: #299.