Basic Design Time Data Context

I figured out how to do a design-time data context. The problem I was
running into was that if a data context was defined the "normal" way in
the xaml, it would look right, but would be an unchanging instance at
run-time.  If I didn't define the datacontext at all, any bound UI
elements would look wrong in the designer, but correct at run time. This
code allows a design-time-only instance to be defined so that it looks
correct both ways.