Adds a menu option to a menu page dynamically.
MenubarAddItem( menu-page, new-menu-option, action )
All parameters are required. Menu-page is the menu page that the new menu option is to be added to. New-menu-option is the name of the new menu option. Action is the action to be called by the new menu option.
This action can be called in the Yes Action or No Action of a decision, in combination with MenubarDeleteItem().