Convert PurpleRequestFieldList into a GObject

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.

Also could implement `GListModel`, but it takes arbitrary pointers too, so not right now.

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

Reviewed at https://reviews.imfreedom.org/r/2336/