Add query-attributes for links to roles

For each Link class, add a query-attribute for that link to each of the
E-Types of the role attributes.

* Add `A_Link_Ref_Set`, `A_Rev_Ref_Set`, and `A_Role_Ref_Set`

* Add `e_type.link_ref_attr`

* Add `cls.show_in_ui_T`

* Remove `_Cacher_` and role- and link-cacher children

* Remove setup of `auto_cache_roles` from
  `MOM.Meta.M_Link._m_setup_roles`

* Remove `Link2_Ordered`