[PythonCAD] More undo/redo stuff checked in

[Art Haas]

Hi.

There is now a 'Redo' menu choice, and it does work. So, you can draw
some entities, then 'undo' the drawing, then 'redo' the drawing to
restore them, then 'undo' again, etc, etc. There is still a lot of
debugging printout going on, and I'm certain that the undo/redo stuff is
not error free, but it is nice to see this stuff in the program and
working at last.

Art
-- 
Man once surrendering his reason, has no remaining guard against absurdities
the most monstrous, and like a ship without rudder, is the sport of every wind.

-Thomas Jefferson to James Smith, 1822