By using %mn, menu items can be added at run time. This is of greatest use when
you wish to show a file history or a list of open windows. Items are added one
at a time to the end of the pop down menu that contains the handle.
In %mn (see Menu format)
an asterisk (*) must be added where a text description would previously have
been placed and a integer argument, instead of a call-back, must be provided to
hold the handle.
handle can then be passed to ADD_MENU_ITEM@.
name is the new string to put into the menu i.e. the new menu item. enable
controls the enabling of the item (1 for enabled, zero for disabled, i.e.
greyed) and the check will add or remove a tick character (1 for checked, zero
for unchecked). A call-back function must be provided so that ClearWin+ can act
upon a menu selection. A separator is obtained if name is input as char(0).