Add a plugin to sort multi-value fields.

The previous commit removes the sorting of multi-value tags for display
only - which improves consistency between what is seen and what is in
the actual ID3 tags. This plugin restores the field sorting in a
consistent way by actually sorting the multi-value fields that are both
displayed and saved.
Improve Metadata Matching / Display and Track Matching

For mp3 files saved with ID3v23 tags (which is common as  Windows does
not support ID3v24) many multi-value tags are flattened. This commit
fixes the situation where this causes mismatches between flattened
original tags and unflattened MB data when these files are reread.