code: func (b *BoxLayout) InsertWidget(index int, widget Widget, fill float64) {
  	c := &boxLayoutCell{
  		widget: widget,
  		fill:   fill,
  		view:   NewViewPort(b.view, 0, 0, 0, 0),
  	}
  	c.widget.SetView(c.view)
  	if index < 0 {
  		index = 0
  	}
  	if index > len(b.cells) {
  		index = len(b.cells)
  	}
  	b.cells = append(b.cells, c)
  	copy(b.cells[index+1:], b.cells[index:])
  	b.cells[index] = c
  	widget.Watch(b)
  	b.layout()
  	b.PostEventWidgetContent(b)
  }
docs: // InsertWidget inserts a widget at the given offset.  Offset 0 is the
  // front.  If the index is longer than the number of widgets, then it
  // just gets appended to the end.