(clicking an item toggles the item on or off independently of other
selections).
-List box elements are numbered from zero.
+List box elements are numbered from zero. Their number is limited in
+some platforms (e.g. ca. 2000 on GTK).
A listbox callback gets an event wxEVT\_COMMAND\_LISTBOX\_SELECT for single clicks, and
wxEVT\_COMMAND\_LISTBOX\_DOUBLE\_CLICKED for double clicks.