[plt-scheme] keybindings

In SVN, I've just made a change to the way keybindings are setup that
might change the priority of some of the keybindings relative to
others. I don't think that there were many actual conflicts, but if
you see that your keybindings aren't working like they used to, please
let me know.

The goal of the change was to ensure that user-defined keybindings
have the highest priority. This means that, amoug others, the
user-defined keybindings can now rebind "[".

Thanks,
Robby