One Relationship Handler to Rule Them All. (Props to solotandem for the wild idea.) Compare the Views subdir to 5a91a18e.

Provide the scaffolding necessary to get endpoint->relation table->endpoint views relationship handlers.

Revert to Views implementation #1 but using a proper relationship handler. TODO: convert relation table joins to proper handlers, too.

Issue #1387936 by boombatower: Cleanup documentation and provide example for relation_relation_exists().

relation_endpoint_field_validate() should also check if entity is_new when testing if the relation is unique. #1350588 fixed by jsacksick.

Views integration: setup relation: type as filter_in_operator instead of string #1365406 by dereine.