[PythonCAD] Second cut at reworking the entity transfer code now on repo

[Art Haas]

Hi.

Once again I've reworked the code for transferring entities between
layers, and my first cut of the code is now at the repo. This reworking
brings the code back closer to the clone() based approach that was used
in the first version, but the code is much simpler and clearer now.

While redoing the code above I found that the problems I saw regarding
undo/redo behavior on dimensions in the reworked code I added to the
latest release has been present for much longer than I'd realized. The
code now at the repo now has abilities for undo/redo operations on
'point_changed' and 'dimobj_changed' Dimension messages that I'd thought
were already in the code but in fact were not present. In addition to
correcting that oversight, I added clone() routines to the various
Dimension entities that, at present, are slightly incomplete. I'll be
adding the needed enhancements this afternoon and weekend, so things
will be more complete on Monday.

For those wanting to test the code out, just do an 'svn update'
and you'll get the changes.

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