[PythonCAD] First bits of undo stuff in the repo

[Art Haas]

Hi.

I've checked them in, and they are very preliminary, but PythonCAD now
has the start of an undo/redo mechanism that actually works! So far all
it does is allow you to undo the creation of an entity, but that is a
start. If you pull the code from the repo and run it, you'll see all
sorts of stuff printed out in the terminal. I checked the code in even
with all the various print-out stuff in place just to get the first blob
of changes in the repo. Many more changes ahead will still be needed to
make undo/redo stuff more useful, but this is a start.

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