[racket] Problem with on-subwindow-event

At Mon, 24 Sep 2012 23:47:17 -0600, Kieron Hardy wrote:
> I used 'enter to make the problem easy to demonstrate, but what I'm really 
> interested in is button up/down events and they get duplicated also. 

It turns out that enter/leave events are generated very differently
from button up/down events. Happily, fixing the duplicated button
up/down events on Windows controls turns out to be easy, and I've
pushed that repair.

I improved enter/leave events, too, but those are less reliable across
platforms for a variety of reasons.