Custom Handlers for Listers: parent

This event will be received when the Parent Directory item is chosen from
the lister cache pop-up menu, or whenever the user clicks on the border
parent gadget or uses the parent hot key, "/".

The message arguments are:-

        Arg0 - "parent"            (a string indicating the event type)
        Arg1 - <handle>            (source lister handle)
        Arg2 - <path>              (path of lister)
        Arg3 - undefined
        Arg4 - undefined
        Arg5 - undefined
        Arg6 - <qualifiers>        (string indicating qualifiers pressed
                                   - only shift keys)