Basic summary information for duplicates

Added a one-line summary of each album (in lib, and import target) so
you can easily tell if (for example) you are about to overwrite your
FLAC copy with a low-bitrate mp3 from somewhere else.
Yeah, eventually—we're already using this in a minimal way with the album/singleton distinction and with the unarchiving task. Thanks for looking at this; it should make the whole ImportTask thing easier to manage.