Custom Handlers for Listers: doubleclick

This is a double-click event, and indicates that an item in the lister has
been double-clicked on by the user.

The message arguments are:-

        Arg0 - "doubleclick"       (a string indicating the event type)
        Arg1 - <handle>            (lister handle)
        Arg2 - <name>              (entry name)
        Arg3 - undefined
        Arg4 - undefined
        Arg5 - <userdata>          (if userdata was specified with the
                                    lister addstem  command)
        Arg6 - <qualifiers>        (string indicating qualifiers pressed
                                   - shift, alt, and control keys)