Panel code reorganization contributed by David Megginson:

Panel code reorganization contributed by David Megginson:

I've reorganized the code in panel.cxx and panel.hxx so that it will
be a little easier to extend the panel later if someone wants to.
It's still basically Friedemann's code at the core, but I've
repackaged it into a saner class hierarchy and encapsulated as much as
I could (there are still a couple of circular dependencies that need
swatting).  If someone wants to modify it to use SSG or to add new
gauges, it should be a lot easier now.

There are no user-visible changes.