Use RcBox for PurpleRequestCommonParameters

Also, stop returning the input from `purple_request_cpar_unref`.
That's not what `g_object_unref` does, and nothing uses the return value.

Also also, just do the cleanup directly instead of calling `purple_request_cpar_set_extra_actions(cpar, NULL);`.

Testing Done:
Ran `Request->Fields` from the demo protocol.

Reviewed at https://reviews.imfreedom.org/r/2308/