set-string in list-box%

Changes an item in the list box.

• (send a-list-box set-string n label) -> void
     n : exact non-negative integer
     label : string (up to 200 characters)

  Sets the item indexed by n. List box items are indexed from 0. If n is equal to or larger than the number of choices, an exn:application:mismatch exception is raised.