code: func (b *BoxLayout) AddWidget(widget Widget, fill float64) {
  	c := &boxLayoutCell{
  		widget: widget,
  		fill:   fill,
  		view:   NewViewPort(b.view, 0, 0, 0, 0),
  	}
  	widget.SetView(c.view)
  	b.cells = append(b.cells, c)
  	b.changed = true
  	widget.Watch(b)
  	b.layout()
  	b.PostEventWidgetContent(b)
  }
docs: // AddWidget adds a widget to the end of the BoxLayout.