Consider deprecating metainfo.creator

[benkiel]

See discussion in #192. This is now optional, it may not have value or a clear "what counts as a creator when a tool may just touch a subset of a UFO" anymore.

Discuss pro/con below

[madig]

I suppose there are three use-cases:

1. A tool saves a fresh UFO (different location or by erasing the existing UFO completely)
2. A tool saves a subset of a UFO
3. A tool just reformats a UFO partially or completely, potentially rewriting some data (e.g. floating point precision)

For case 1, the creator is obviously the tool. For case 2, I don't think there can be a clear answer. Maybe the field degrades to "creator of the initial UFO", whatever that means? For 3. I suppose the creator can stay unchanged if all the tool changes is whitespace. For rewritten data, we are potentially back at case 2...

One hard-and-fast rule that somewhat makes sense is to have whatever tool last touched the UFO in any way have set itself as the creator, making it more like a lastModifiedBy.

Another is to only change the field when writing out the metainfo.plist file. So creator would be "last tool to write this UFO out fresh".