[Fixed Position] Expose view proxy

Expose view proxy as selection state from the Fixed Position
controller, to make Add button available under generalized
selection mechanism, WTD-929.

platform/features/layout/src/FixedController.js

@@ -208,9 +208,14 @@ define(
                 });
             }
 
-            //  Track current selection state
+            // Track current selection state
             selection = $scope.selection;
 
+            // Expose the view's selection proxy
+            if (selection) {
+                selection.proxy(new FixedProxy(addElement, $q, dialogService));
+            }
+
             // Refresh list of elements whenever model changes
             $scope.$watch("model.modified", refreshElements);