Initial Key field support [Feature Request]

[colin-marshall]

A big headache for DJ's is that most DJ software uses the Initial Key tag field but most media players like iTunes don't make it available. Would be great if there was an option to show the initial key field in a column and in the "Get Info" window. It could simply be labeled "Key"

[jcm93]

I'm open adding new fields like this one, but I'm unsure on the implementation details at the moment.

Ideally, I'd like to keep the set of tags I've got as the stock set, and introduce a system by which users can have their own custom fields which are just as fast/performant as the stock set, but that's going to be moderately complex.

At a minimum, If I just added a field to the Track table/class, I'd like to sort out how the Core Data versioning system works so I can perform clean migrations from one version of jmc to another when the data model changes. It will be much better for the future if I stop making ad-hoc changes the schema and figure out how to version things properly. Gonna table this at least until I do that.