bug#50752: 28.0.50; easy-menu-define lowers the menu-bar key

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Wed, 13 Oct 2021 17:26:33 +0200
> Cc: Eli Zaretskii <eliz@gnu.org>, Shuguang Sun <shuguang79@qq.com>, 
> 50752@debbugs.gnu.org
> 
> Do we have an alternative to downcase, or should we just ensure that
> it uses a standard case-table?  Could that lead to any other problems?
 
We could use the equivalent of

  (get-char-code-property ?I 'lowercase)

If the above returns nil, it means the lower-case variant is the
character itself.

In C, this means to use uniprop_table, like bidi.c and casefiddle.c
do.  This accesses the database generated from UnicodeData.txt.