Finish what was committed in a broken state yesterday.

Finish what was committed in a broken state yesterday.
Split up main.cxx into a program manegement part (which remains in
main.cxx) and a render part (the new renderer.?xx files). Also turn
the renderer into a small class of it's own. At this time not really
exctining because most of the stuff is still global, but it allows us
to slowly migrate some of the global definitions into the new class.

The FGRenderer class is now managed by globals, so to get the renderer
just call gloabals->get_renderer()

At some pijt it might be a good idea to also turn the remaining code in
main into a class of it's own. With a bit of luck we end up with a more
robust, and better maintainable code.