Support library deduplication

[kepstin]

Detect when multiple copies of the same recording are available, and notify the user. Optionally allow selecting a preferred version that will be used in place of another version - e.g. because it's higher quality.
Support shuffling recordings rather than file, and pick the best file if multiple files are available for a recording.
Also important: support a mode where when you're playing a release in order, it will always use the file from that release rather than an alternate version (e.g. for gapless or album replaygain)