This control is similar to %`ls but cur_item is a string for the current
text rather than the index of the selected item.
If a call-back is supplied, option can be set to the keyword extended.
The result is that the call-back is called when the list drops down or closes
up. These events must be detected from the call-back by calling
clearwin_string@ with the argument 'CALLBACK_REASON' which returns
'DROPDOWN' or 'CLOSEUP'.
Using clearwin_info@ with the argument 'EDITABLE_LISTBOX_INDEX' provides the
index of the list-box item that has been selected by the user. If this value is
obtained within the %el callback function then you may need to add the option
%el[extended] in order to ensure that the callback function is called
every time an item is selected.
See %ls for further details.