private void Unsubscribe (IBrowserEventSubscriber subscriber)
 {
   DragEnter -= subscriber.OnDragEnter;
   DragDrop -= subscriber.OnDragDrop;
   DragOver -= subscriber.OnDragOver;
   DragLeave -= subscriber.OnDragLeave;
 }
 private void Subscribe (IBrowserEventSubscriber subscriber)
 {
   DragEnter += subscriber.OnDragEnter;
   DragDrop += subscriber.OnDragDrop;
   DragOver += subscriber.OnDragOver;
   DragLeave += subscriber.OnDragLeave;
 }