Item Redraw Performance - Bug Fix (#3513)

* Fixing a bug with the call to `repositionX`. The `items` collection is an object, not an array, so it cannot be iterated over using a normal for loop. Not repositioning the items causes the vertical stacking to jump around sporatically while scrolling.

* Update Group.js

lib/timeline/component/Group.js

@@ -300,9 +300,9 @@ Group.prototype._redrawItems = function(forceRestack, lastIsVisible, margin, ran
         }
       }
 
-      for (i = 0; i < this.items.length; i++) {
-        this.items[i].repositionX(limitSize);
-      }
+      util.forEach(this.items, function (item) {
+        item.repositionX(limitSize);
+      });
 
       if (this.doInnerStack && this.itemSet.options.stackSubgroups) {
         // Order the items within each subgroup