racket/gui: fix reporting of numpad Enter as 'numpad-enter

On Unix and Mac OS X, the key has long been reported as #\u3,
even though the documentation says 'numpad-enter.

A more compatible change would be to adjust the documentation and
also change DrRacket and the framework to recognize #\u3 instead
of 'numpad-enter. But there doesn't seem to be a good reason to
treat #\u3 as "numpad Enter" --- particularly as opposed to treating
it as "Ctl-C" --- other than ancient Mac history.

On Windows, numpad Enter continues to be reported as #\return,
since Windows event handling doesn't naturally or easily distinguish
the two keys.
DrRacket: drop numpad Enter treatment as force-evaluation

DrRacket's interactions window tried to treat numpad Enter the same
as ctl-Return: send input to the reader, even if an S-expression is
incomplete. Since numpad Enter events were not reported as
'numpad-enter, however, the special treatment didn't take effect.

Meanwhile, it appears that some keyboard configurations report a
numpad Enter event when NumLock is on and the regular Enter key
is pressed, and that's probably not what a user wants. And
numpad Enter has never been reported differently from regular
Enter on Windows.

So, it's simpler and more consistent to just treat numpad Enter
and regular Enter the same.