Add PurpleRequestField.is_filled vfunc, and a filled property

And then move the string-specific check into the subclass.

Note that this just does a `g_object_notify(G_OBJECT(field), "filled");` instead of adding a convenience function; not sure if I should do that.

Testing Done:
Compiled and ran `ninja test`.

Reviewed at https://reviews.imfreedom.org/r/2345/