Prevent redirect to blank after drag and drop (FF) (#2871)

Fix proposal for issue #2842

More details here : #2842

lib/timeline/Core.js

@@ -245,6 +245,9 @@ Core.prototype._create = function (container) {
   }
 
   function handleDrop(event) {
+    // prevent redirect to blank page - Firefox
+    if(event.preventDefault) { event.preventDefault(); }
+    if(event.stopPropagation) { event.stopPropagation(); }
     // return when dropping non-vis items 
     try {
       var itemData = JSON.parse(event.dataTransfer.getData("text"))