fix: #2422 Timeline onMove callback (#2427)

* Fix redraw order
* Fix calling dragStart mote than once

lib/timeline/component/ItemSet.js

@@ -1213,6 +1213,7 @@ ItemSet.prototype._getGroupIndex = function(groupId) {
  * @private
  */
 ItemSet.prototype._onDragStart = function (event) {
+  if (this.touchParams.itemIsDragging) { return; }
   var item = this.touchParams.item || null;
   var me = this;
   var props;