Add a PurpleRequestFieldString subclass

This also does an `hg cp`, though with all the renaming of the parameter names, maybe that wasn't as useful for tracking the diff.

Note, I didn't bother re-indenting some of the blocks, because they'll all eventually be moved when everything is subclassed.

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

Reviewed at https://reviews.imfreedom.org/r/2324/