Add compose sequences for acute-accented Cyrillic vowels

The acute accent is commonly used to mark syllable stress in dictionaries, encyclopedias, and language learning materials for Russian and some other Cyrillic-script languages. The accent is placed over the vowel of the syllable with primary stress. (See for example the Russian editions of Wikipedia and Wiktionary, or virtually any printed Russian dictionary or printed textbook for learning Russian.) However, libX11 is missing such compose sequences for many Cyrillic vowels, which makes it difficult to compose lexicographic or language learning material. One can always create a custom $HOME/.XCompose, but this is not easy, particularly for users that are not technically inclined. It would be better if these compose sequences were available system-wide by default.

Among vowels used in Russian, the current libX11 compose sequences cover а, у, о, е, and и, but not э, ы, я, or ю. (The vowel ё is not covered either, but at least in Russian it is always stressed and so never marked.) Other languages, such as Ukrainian, have further vowels that I know sometimes take acute accents. However, I lack detailed and comprehensive knowledge of all such languages (that is, what vowels they use, and whether or not it is common practice for them to be marked with an acute accent in pedagogical and lexicographic sources) and so have not checked whether their compose sequences are also missing.

I will prepare and submit a merge request that covers at least Russian; if it is accepted then perhaps this issue can be left open in the event that someone wants to contribute the necessary sequences for Ukrainian and other Cyrillic-alphabet languages.

Note: This issue was previously reported for XKB, first as Bug 2793 on the old Bugzilla and then as Issue 184 here on GitLab. In those bugs the consensus was that this is a libX11 issue, not an XKB issue.