artwork: MusicBrainz back end, fixes for lookups without metadata
vfs_curl: another try to get errno=ECONNABORTED on aborts
artwork: better handling of aborts, don't create a placeholder and exit immediately
coverart: make image changes due to cache expiry and invalidation smoother
artwork: musicbrainz module copyright
coverart: speed up the most common get_album_art pathway
artwork: be nicer when no lookup sources are specified, but coverart widgets are present
coverart: use the same pixbuf cache entry for all widths for the default image
artwork: use musicbrainz release groups for more complete results
artwork: simpler flood control checks and error handling
coverart: handle aborts and other situations where the cache file goes missing
artwork: thread safe http aborts, cache reset edge cases
artwork: cache reset timings and thread safety
Merge pull request #49 from Lithopsian/search

artwork: MusicBrainz plus various fixes