fixed memory leak because eventlisteners weren't destroyed (#3728)

lib/timeline/Core.js

@@ -527,6 +527,7 @@ Core.prototype.destroy = function () {
     }
   }
   this.listeners = null;
+  this.hammer && this.hammer.destroy();
   this.hammer = null;
 
   // give all components the opportunity to cleanup

lib/timeline/component/ItemSet.js

@@ -483,6 +483,8 @@ ItemSet.prototype.destroy = function() {
   this.setItems(null);
   this.setGroups(null);
 
+  this.hammer && this.hammer.destroy();
+  this.groupHammer && this.groupHammer.destroy();
   this.hammer = null;
 
   this.body = null;