[Containment] Expose Link action as compose

Expose 'link' as 'compose'; this facilitates implementation of
drag-drop composition being deferred to actions which will
support move/copy/link options in the future. WTD-962.

platform/commonUI/edit/bundle.json

@@ -31,6 +31,10 @@
             }
         ],
         "actions": [
+            {
+                "key": "compose",
+                "implementation": "actions/LinkAction.js"
+            },
             {
                 "key": "edit",
                 "implementation": "actions/EditAction.js",