Advanced search and upgrade to local backend to support it.

Advanced search allows you to search more efficiently for albums and artists,
without going through the whole list of tracks.

This change adds advanced_search support to the core, and to the local backend.
Other backends should fall-back to the old search functions.

The local json backend is upgraded to store the same data in a sqlite memory
database. It makes searching way more efficient, and much less fragile.

Searches also return albums and artists now.