code: protected MaterialDnd buildDnd() {
          MaterialDnd dnd = MaterialDnd.draggable(this, buildDragOptions());
          dnd.ignoreFrom(".content, .window-action");
          return dnd;
      }
docs: Override to provide custom {@link MaterialDnd} instance. Default implementation will construct {@link MaterialDnd}
  using options provided by {@link #buildDragOptions()} and will ignore drag events from content portion of
  the window ({@link AddinsCssName#CONTENT}) as well from action buttons (close, maximize and other {@link AddinsCssName#WINDOW_ACTION}.