[PythonCAD] Updates in repo

[Art Haas]

Hi.

The latest updates available at the repo contain numerous changes. The
display of the various layers in a drawing is now done by the
LayerDisplay class in the 'gtkshell.py' file, the first utilization of
code in this relatively new file. The code is much cleaner and easier
to understand than the hodge-podge that had been in the GTKImage class,
so not only are things easier to follow the change allowed the removal
of a great deal of code from GTKImage. Also, the GTK event handling code
has begun to get a long needed review. PythonCAD has not been a good
PyGTK or GTK client in some ways, as often the event handling code
returned values unnecessarily. The handling of the 'activate' signal
by menuitem widgets was a prime example. The first round of cleanups has
removed the unnecessary return statements and I've started fiddling with
handlers of 'motion_notify', 'key_press', 'key_release', and the button
press/release so they return Boolean values. More of this work is
expected over the coming weeks/months.

A few bug fixes are also included, especially in the compatability code
in the gtkactions.py file. A few quirks were addressed - the undo-ing of
a split on a segment entity would usually require two undo menu choices,
now it is done (correctly) with one menu choice. Also, stretch
operations that selected the stretched entities with a box will now were
stored in the undo/redo chain as individual events instead of a single
action, and that has been fixed. Other oddments here and there have been
addressed as well.

Enjoy.

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