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 <jfelder@src.gnome.org>.

Related: #299.
5 jobs for wip/mschraal/core-squash in 8 minutes and 26 seconds (queued for 3 seconds)
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