Custom Handlers for Listers: AddStem Pop-Ups

If you have added files to a lister with  lister addstem  and specified
your own pop-up menus for the files, you will receive messages when these
menus are chosen by the user.

The message arguments are:-

        Arg0 - "menu"              (string identifies this as a menu event)
        Arg1 - <handle>            (lister handle)
        Arg2 - <name>              (entry name)
        Arg3 - <id>                (ID of the menu item + base ID if specified)
        Arg4 - "file"              (string identifying this as a "file" menu
                                   event)
        Arg5 - <userdata>          (userdata specified via the
                                    lister addstem  command)