[PythonCAD] Automatic entity splitting now in PythonCAD

[Art Haas]

Hi.

This is a feature I've wanted PythonCAD to have since the project began.
Draw a segment, and then draw a second segment where one of the
endpoints is somewhere on the first segment. The first segment will
be replaced by two segments sharing the endpoint of the newly created
segments, so now the drawing has three segments. Similarly, adding a
new point to a drawing which lands on a Circle will split it into an
Arc, a new point on an Arc splits it into two Arcs, and a new point
on a Polyline just gets added to the Polyline point list. The autosplit
feature can be turned on and off via the Preferences dialog if you
don't want to use this new feature.

Those of you using Subversion can try this new code out by simplying
updating your code via 'svn update'. Let me know what you think.

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