First baby steps towards restructuring the view architecture. I've

First baby steps towards restructuring the view architecture.  I've
created a new class, FGViewPoint (declared in viewer.hxx) that holds a
single position in FlightGear coordinates, and have used it to factor
a lot of the common code out of viewer_lookat.cxx and viewer_rph.cxx.

I don't know whether this new class will stay or not; it might just be
a temporary step, or it might end up taking over much of the current
viewer functionality.  It would be a bad idea to code against it right
now.