Sets the relation_type property to relation_type such
as GCONTACT_EVENT_ANNIVERSARY or
GCONTACT_EVENT_OTHER.
If relation_type is null, the relation type will be unset. When the
GContactEvent is used in a query, however, exactly one of
relation_type and
label must be null.
| this | |
| relation_type |
the new relation type for the event, or |