Handle Performer tags stored in ID3v2.3

Performer tags are stored as part of the IPLS tag in ID3v2.3 and this is
put into the TIPL tag when read by mutagen. This fix adds unknown TIPL
roles as Performer tags.
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.
Make multi-value tagging in ID3v23 consistent with Picard display.

There is no agreed standard between taggers for how multi-value tags are
saved in ID3v23 files - we can choose which separator to use. This
change makes the separator consistent with that used in metadata.py.