Add Author/Source metadata field for patch

[christofmuc]

Related to my comments in #379, I would like to see at least one additional field for author / source (soundpack).

Yes, I could use comment for that, but I'd rather leave comment as ad-hoc freetext. Author / source seems to me as relevant / important / meaningful property of each patch and is easy to understand for majority of users.

Some might prefer comment 1, comment 2 and comment 3 and define their own approach how to handle metadata, but that would be less intuitive and more difficult to manage, especially if sharing patches with other users etc.

Originally posted by @RadekPilich in #71 (comment)

[christofmuc]

@radekpilich I like that, that's a quite straightforward solution! We need to think where to put the UI to populate the field, and if there should be any auto-population done during import.