library: Add empty library implementation
playlists: Set bitrate on tracks in looked up playlists
library: Add track lookup
library: Add album lookup
library: Add artist lookup

First looks up artist, then for each of the artist's albums, tracks are looked
up. Fixes #7.
library: Make lookup handle invalid URIs
library: Make lookup log unhandled link types
library: Add playlist lookup
library: Split lookup impl into multiple functions
library: Filter out tracks which are None