[plt-dev] DrScheme definitions crash in pre-release

Thanks. I think I've fixed this now. The problem was that Check Syntax
was not coping well with modifications to the editor while it was also
updating the editor (and saving modifies the definitions window in the
teaching languages). I've now made it cope with them a bit better, but
I am not confident that my changes haven't introduced some more subtle
problem, so I prefer to leave this alone for the release (since the
bug has been there for years two more months won't matter).

Robby