racket/gui win32: fix Space handling of buttons and checkboxes

Suppress WM_KEYDOWN messages to a button or checkbox, so that the
top-level window takes care of Space and Return for a button or
checkbox with the keyboard focus.

Closes PR 14710
racket/gui keymap: adjust handling of 'control, etc., events

If a keymap has a grab, then discard key events that are modifier
changes or key releases.

Adjusts commit 8ba8d01ec0 so that Ctl-u work in DrRackt (and
other framework-based programs).