[plt-scheme] fixed a Windows bug --- maybe the performance problem

Based on recent reports about problems under Windows XP, I took a close
look at the code that runs Windows built-in dialogs, such as the
get-file dialog.

I found a problem, and I've exp-tagged a fix in CVS. For those using
Windows, please let me know

  1. if you were seeing performance problems before, whether the latest
     solves anyhing;

  2. if there's no improvement, whether you see problems even before
     opening a dialog; or

  3. if things seemed to work fine before, whether things still seem
     fine.

Query #3 is because I changed some code that seems, now, completely
wrong. It's possible that I knew something three years ago that I don't
remember. (I think, however, it's far more likely a stupid mistake.)

Matthew