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/
# UPDATING VERSION NUMBERS FOR RELEASES
#
# The version number is:
# <major>.<minor>.<micro><extra>
#
# micro += 1
#
# If any functions have been added to libpurple, Pidgin, or Finch:
# micro = 0
# minor += 1
#
# If backwards compatibility has been broken in libpurple, Pidgin, or Finch:
# micro = 0
# minor = 0
# major += 1
# purple_soversion += 1
#
# extra should be similar to one of the following: