js: Be more subtle about cancelling keydown events

Now we only cancel a keydown event if the C keyboard handler recognises
the key and passes it on to the midend.  This doesn't necessarily mean
that the midend has actually done anything with it, of course.  Still,
this is enough to allow F12 to open the developer tools even when the
input focus is on the puzzle.  It also allows for tabbing out of the
puzzle into the links below it.

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