Switch to registry for most of mopidy extension hooks

Fixes #601

- [ ] Changelog still missing
- [ ] Docs for registry should be updated to help people start using this
- [ ] Test for #500 needs to be reworked.

I was also considering moving commands into this, but it gets complicated from a bootstrap point of view. Otherwise I considered moving the ``read/write_library`` bits into models, but that can wait. Lastly, we should probably create a at the very least a local library cookie cutter, or the first sample one to get us started.