AtkRelation defines the functional relationship between two #AtkObjects Defines the functional relationship between two #AtkObjects. @ATK_RELATION_NULL: @ATK_RELATION_CONTROLLED_BY: @ATK_RELATION_CONTROLLER_FOR: @ATK_RELATION_LABEL_FOR: @ATK_RELATION_LABELLED_BY: @ATK_RELATION_MEMBER_OF: @ATK_RELATION_LAST_DEFINED: @name: @Returns: @targets: @n_targets: @relationship: @Returns: @relation: @Returns: @relation: @Returns: