In order to make menu selections visible on older terminals without highlighting or color capability, the menu library marks selected items in a menu with a prefix string.
set-menu-mark! sets the mark string for the given
set-menu-mark! with a null menu item will abolish
the mark string. Note that changing the length of the mark string for
a menu while the menu is posted is likely to produce unhelpful
The default string is “-” (a dash).
Returns, as a string, the prefix string used by the menu library to designate the selected item.