[macCreatorType] preserve existing other metadata when setting filetype & creator

[NSGod]

This addresses #3170.

I'll just note that I'm not fluent in Python, so I don't know if there'd be a better way to fix this than what I came up with.

[anthrotype]

thanks, this looks good, though I haven't tested it. I wonder how difficult it'd be to add a test.

Also it doesn't look like this setMacCreatorAndType function is used anywhere in fontTools... Anybody remembers why it's there? /cc @justvanrossum

@NSGod out of curiousity, what do you use this method for?

[justvanrossum]

Anybody remembers why it's there?

It quite a legacy thing for MacOS Classic. I found two spots in old fonttools where it was used, and rightfully removed later. I can't find other references to it.

[NSGod]

@NSGod out of curiousity, what do you use this method for?

Well, I haven't actually used fontTools all that much yet, or that code at all, for that matter (still learning Python). I've written software that converts between the older formats and newer ones, so I'm keenly aware of the importance of the com.apple.FinderInfo. It was kind of an effort to force myself to learn Python.

While the type and creator code are no longer important, the rest of the info includes things like file label and custom icon bit might be worthwhile to preserve if overwriting files.

[anthrotype]

so it looks like neither us nor yourself are using that piece of code, I am tempted to just get rid of it or deprecate it/leave as is and if one cares about retaining those attributes they can implement this in their own code..