[Core] Edit-wrap relationship capability

Wrap objects retrieved via the relationship capability
with Edit mode caching etc, for WTD-1007.

platform/commonUI/edit/src/capabilities/EditableRelationshipCapability.js

@@ -0,0 +1,36 @@
+/*global define*/
+
+
+define(
+    ['./EditableLookupCapability'],
+    function (EditableLookupCapability) {
+        'use strict';
+
+        /**
+         * Wrapper for the "relationship" capability;
+         * ensures that any domain objects reachable in Edit mode
+         * are also wrapped as EditableDomainObjects.
+         *
+         * Meant specifically for use by EditableDomainObject and the
+         * associated cache; the constructor signature is particular
+         * to a pattern used there and may contain unused arguments.
+         */
+        return function EditableRelationshipCapability(
+            relationshipCapability,
+            editableObject,
+            domainObject,
+            cache
+        ) {
+            // This is a "lookup" style capability (it looks up other
+            // domain objects), but we do not want to return the same
+            // specific value every time (composition may change)
+            return new EditableLookupCapability(
+                relationshipCapability,
+                editableObject,
+                domainObject,
+                cache,
+                false // Not idempotent
+            );
+        };
+    }
+);