Called when this window or a child window receives a keyboard event. The top-most parent's pre-on-char method is called first; if the return value is #f, the pre-on-char method is called for the next child in the path to the receiver. Finally, if the receiver's pre-on-char method returns #f, the event is passed on to the receiver's normal key-handling mechanism.
The event argument is the event that was generated for the receiver window.