Add a PurpleRequestField:valid property and is_valid vfunc

* Add a `PurpleRequestField:valid` property and `is_valid` vfunc.
* Add a `GDestroyNotify` parameter for the validator data, and document the callback.
* Correct the property notification in `purple_request_field_bool_set_value`.

Testing Done:
Compiled and ran `ninja test`

Reviewed at https://reviews.imfreedom.org/r/2346/