Support for Musichi classical fields

[IJabz]

Although this s/w doesn't seem anything special it does define some classical fields that maybe should be supported, reused

For ID3 we have additional TXXX frames as follows:

• TXXX:Composition

• TXXX:Instrument

• TXXX:Period

I think composition is analogous to a top level MusicBrainz work

Period is a date period like Baroque, Medieval, could be useful

Instrument is a list of instrument with time signatures but no performers so limited use

It also has custom tags but tied to the player so ignore these such as

TXXX:MCHICUST1

TXXX:MCHICUST2 ...

[IJabz]

Added Period, Instrument is fairly useless IMO and I wouldn't make it a standard field. Composition is probably going to be same as Work, I think would prefer to stick with work as the standard field since I think only Musichi uses Composition field.

[IJabz]

Done, just added Period field in the end.