- Improvements in support for non-ASCII (European) keysyms under X

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If a user has a keyboard with known standard non-ASCII character
equivalents, typically for European users, then Emacs' default
binding should be self-insert-command, with the obvious character
inserted.   For example, if a user has a keyboard with

xmodmap -e "keycode 54 = scaron"

then pressing that key on the keyboard will insert the (Latin-2)
character corresponding to "scaron" into the buffer.

Note: Emacs 20.6 does NOTHING when pressing such a key (not even an
error), i.e. even (read-event) ignores this key, which means it can't
even be bound to anything by a user trying to customize it.

This is implemented by maintaining a table of translations between all
the known X keysym names and the corresponding (charset, octet) pairs.

/* For every key on the keyboard that has a known character correspondence,
   we define the ascii-character property of the keysym, and make the
   default binding for the key be self-insert-command.

   The following magic is basically intimate knowledge of X11/keysymdef.h.
   The keysym mappings defined by X11 are based on the iso8859 standards,
   except for Cyrillic and Greek.

   In a non-Mule world, a user can still have a multi-lingual editor, by doing
   (set-face-font "...-iso8859-2" (current-buffer))
   for all their Latin-2 buffers, etc.  */