[plt-scheme] extending gui.ss

> The gui.ss teachpack is by far the worst software I ever designed and
> released. Mea culpa. It'll go away. -- Matthias

I hope you mean that it will be redesigned, because it is in keeping with
your idea of attracting students by having them create small versions of
things that excite them about computers, and it serves me well at two
points in my course: as a good use of functions as arguments (as done in
HtDP) and as a more "realistic" example of a good use of mutation (as done
by Matthew in his calculator examples). I think students would feel its
loss.

Matthew's suggestion, which should have been obvious to me, works just
fine to fix my problem. Thanks to both of you. --PR