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.
Handle multi-value tags in ID3v2.3 files

For a specific set of tags or TXXX sub-tags, split single value tags
with a '/'.
Handle id3v2.3 tags by unflattening final

id3v2.3 tags are flattened i.e. multi-value fields are concatenated.
When you reload the tags do not match the unflattened ones from MB. This
fix splits these fields at the concatenation characters - which have
been changed from '/' to '; ' to improve functionality.