Fix frame preview toggle when moving

The hiddenFrames list is not updated correctly when a frame is moved.
Unmoved frames are mapped incorrectly and result in "undefined" entries
in the list, and the resuling list is left unsorted. Make sure unmoved
entries still return a value when mapping and sort the list after frames
have moved.

src/js/controller/piskel/PiskelController.js

@@ -214,7 +214,10 @@
           return index + 1;
         }
       }
-    });
+
+      return index;
+    }).sort();
+
   };
 
   ns.PiskelController.prototype.hasVisibleFrameAt = function (index) {