listbox.go
func (l *ListBox) AddItem(item string) bool {
	l.items = append(l.items, item)
	return true
}
func (l *ListBox) AddItem(item string) bool {
	l.items = append(l.items, item)
	return true
}
// own methods
// AddItem adds a new item to item list.
// Returns true if the operation is successful