code: public static void makeDraggable (HasAllMouseHandlers dragHandle, PopupPanel target)
      {
          DragHandler dragger = new DragHandler(target);
          dragHandle.addMouseDownHandler(dragger);
          dragHandle.addMouseUpHandler(dragger);
          dragHandle.addMouseMoveHandler(dragger);
      }
docs: Adds mouse handlers to the specified drag handle that cause the supplied target popup to be
  dragged around the display. The drag handle is assumed to be a child of the popup.