[plt-scheme] changes to editor<%> undo

In SVN, the default max undo-history for an editor<%> is now 0 instead
of 256. As a result, editing must be explicitly enabled for an
editor<%> using the `set-max-undo-history' method; supply 'forever as
the argument to allow essentially unlimited undo.

The old 256 default was too small to be right for most end-user tasks,
and it was too large to be right for other editor<%> tasks. For the
former, disabling undo by default makes it more obvious when a large
undo history should be enabled. For the latter, disabling undo prevents
confusing snip behavior and unnecessary memory consumption.


Meanwhile, users who prefer Emacs-style undo can now enable it (for all
MrEd applications, including DrScheme) by manually adding

 (|MrEd:emacsUndo| #t)

to the PLT preferences file.


Matthew