Use string.ascii_letters instead of string.letters (SF bug #226706)

"Add Alt-slash to Unix keydefs (I somehow need it on RH 6.2).
Get rid of assignment to unused self.text.wordlist."  --GvR

Much has changed -- too much, in fact, to write down.
The big news is that there's a standard way to write IDLE extensions;
see extend.txt.  Some sample extensions have been provided, and
some existing code has been converted to extensions.  Probably the
biggest new user feature is a new search dialog with more options,
search and replace, and even search in files (grep).

This is exactly as downloaded from my laptop after returning
from the holidays -- it hasn't even been tested on Unix yet.

Features: text editor with syntax coloring and undo;
subclassed into interactive Python shell which adds history.