Possible new fields to add:
But need corresponding PRODUCER field etc, because for ID3 just get written to IPLS/TIPL
Made necessary changes in jaudiotagger (#333).
Existing PRODUCER field will now automatically use TXXX:PRODUCER
But we need to add support back in for adding PRODUCER to INVOLVEDPEOPLE field for ID3
Added new sort fields, includes adding to spreadsheet and creating a new People Advanced tab for Manual Edit