Linked tracks need bookmark to persist outside of music directory

[NattyNarwhal]

See discussion in GH-196; easiest way is to persist bookmarks somehow and recognize when they go stale. We could put it in an attribute for the music item/track entity in Core Data, but that seems fragile. A lot of software like Cog or Sequel Pro uses a view in preferences to manage allowed directories. We could perhaps get consent there and store it in i.e. the defaults database.

That said, I'm also unsure if we want to emphasize linked tracks or the local library in general, but that's a different discussion.