Adds a new item to the list of user-selectable items. The current selection is unchanged (unless the list control is an empty choice control, in which case the new item is selected).
Adds a new item to the list box with an associated ``data'' object. The data object is not displayed in the list box; it is provided merely as a convenience for use with get-data, possibly allowing a programmer to avoid managing a separate item-to-data mapping in addition to the list box control.