New field: Completionist / CompletionDate

[thijzert]

Currently there's no way to canonically store who completed the work, if someone other than the composer did so.
Hacks include storing both names in the composer field (e.g. Mozart/Süssmayr), or adding the completionist as some sort of a performer.

A Completionist field (with a data type analogous to Composer) would solve this problem neatly.
When used in conjunction with a new field CompletionDate, we can mark the year in which the sketch was written separately from when that sketch was completed.