Unicode keysyms under X

[Dave Love]

I don't know whether/where it's documented, but recent X versions seem
to support unicodes as keysyms of the form `Unnnn' and 0x0100nnnn for
character U+nnnn.  (I assume they're not actually confined to the
BMP.)

These ought to be decoded in `handle_one_xevent'.  I assume you can't
run `decode-char' there, so I guess it would have to be done by
explicit lookup in the translation tables if the code isn't in the
mule-unicode charset ranges.