Coverart, thread safety, pixbuf cache

Separate pixbuf caches for the widget (one entry) and playlists (20 entries).  New API needed to use a particular cache, bump minor version.  Old API uses the playlist cache.
Sort the cache for faster retrieval, especially when resizing.
Use CAIRO_FILTER_BEST for default pixbuf.
Improved mutex locking for queue and cache management to solve very infrequent hangups and dropped draws.
Dynamic callback list instead of fixed array, reduced memory for most cases, callbacks never dropped.