Jean Felder authored May 13, 2020 and Victor Toso committed Jul 22, 2020

It is the counterpart of grl_data_set_for_id() for GrlRelatedKeys (see
commit b30608ab).

grl_related_keys_set_for_id() allows keys that aren't registered,
to be registered and set it's value. It simply sets the value
for registered keys.

grl_registry_register_metadata_key_for_type() and
grl_registry_register_or_lookup_metadata_key() need to be updated to
add a new bind_key parameter. For related keys, bind_key parameters is
used to create a relation between the key and bind_key.