bug#36717: 25.3; greek.el: deprecated vowel+oxia combinations should be replaced with vowel+tonos counterparts

[Robert Pluim <rpluim@gmail.com>]

>>>>> On Fri, 19 Jul 2019 09:57:38 +0300, Eli Zaretskii <eliz@gnu.org> said:

    Eli> We could ask on the Unicode mailing list.  There are Unicode experts
    Eli> there, and they are quite friendly.  If someone can come up with a
    Eli> comprehensive description of our situation and the issues we are
    Eli> trying to resolve, please write to unicode@unicode.org, and ask the
    Eli> questions.

I think reading <https://www.unicode.org/faq/greek.html> helps
some. My understanding of the situation is that the basic Greek block
should be used, rather than the extended Greek block, for the LETTER +
OXIA/TONOS combinations (and the extended block versions all decompose
to characters in the basic block + a combining mark).

To me that implies that the Greek input methods should use GREEK TONOS
(\u384) consistently rather then GREEK OXIA (\u1ffd), but I couldn't
see any explicit mention of that, and at least in my font they're
visually distinct.

Thereʼs also <http://www.opoudjis.net/unicode/unicode.html>, but
thatʼs much longer, so I only read the bit about oxia vs tonos, and it
also has nothing to say on which to use when inserting only the
accenting character itself.

Robert