registry: fill in elementfactory when registering element

registry: fill in elementfactory when registering element

elementfactory field is filled in by gst_element_base_class_init,
but it needs some info set on the element's type, so have it
available prior to class structure creation spinning up.
This affects elements that have a well-known/public type (e.g. pipeline)
and can be created by other means than gst_element_factory_make
(which will also fill in the element's factory).