Split PurpleRequestFieldGroup into its own files

As before, this uses `hg cp`, so it _looks_ like a bunch of deletion.

Again, there's a bit of private API for the cross-struct access, which should go away later.

I also dropped 'field' from the file name, as it seems like there's no need to have that there in the final class name.

I did not do any other API changes to make this a simpler review.

Testing Done:
Compiled and opened Request Fields from the Demo.

Reviewed at https://reviews.imfreedom.org/r/2316/