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/
/* purple
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