code: func (p *Panel) SetMenu(w Widget) {
  	index := 0
  	if p.title != nil {
  		index++
  	}
  	if p.menu != nil {
  		p.RemoveWidget(p.menu)
  	}
  	p.InsertWidget(index, w, 0.0)
  	p.menu = w
  }
docs: // SetMenu sets the Widget to display in the menu area, which is
  // just below the title.