change in X character input processing

[Dave Love <d.love@dl.ac.uk>]

I've changed the handling of character input under X to consider the
definitions of keysyms rather than trying to decode everything for
some locale which may be inappropriate.  Thus, for instance, my euro
key will work in a Latin-1 environment like the pound key (and doesn't
depend on Latin-9 support in the X libraries).

I think this DTRT generally, but if some sort of input doesn't work
right, please let me know.  There seems to be a shortage of useful
documentation in this area.

Note that a given keysym produces a character from a fixed charset,
e.g. Odiaeresis always produces (make-char 'latin-iso8859-1 86).  The
Kana keysyms produce mule-unicode since I'm not sure how to map them
onto CJK charsets from the defined unicodes; if anyone is sure how to
translate them, please update the table in term/x-win.el which
populates `x-keysym-table'.

Note also that input via a Compose key and other sorts of X input
methods will be different.  Thus with typical XKB rules, `Compose e ='
won't produce € in a Latin-1 locale but `AltGr 4' will.  (That's using
X's Multi_key and Mode_switch.)