[racket] Extending the Racket GUI with native widgets

I see... You need `get-client-handle', which returns the inner widget
of a window rather than the outer one. (There are two layers to a
`panel%' for event-handling reasons.)

I've added `get-client-handle' to `window<%>', which you should use
in this case instead of `get-handle'.

Also, don't forget to use `gtk:widget-show'.