6926: display genitive/nominative months as needed

First proof of concept, fixing the original bug for Russian.
Added gramps/gen/datehandler/_datestrings.py, to hold
all the localized strings, with genitive/nominative
context as appropriate.

Remaining TODO:
1) other languages
2) expand the localized string into RE-style
concat via |, so that displayer uses the 1st instance,
and the parser uses the whole RE. Write the corresponding _().
3) same with short names as with long names
4) harmonize with _grampslocale code reading the names from
elsewhere for Julian/Gregorian months
5) expand to other calendars (non-Julian/non-Gregorian)
6) obsolete the public attributes in dd and replace with accessors

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@23173 4ae1f11a-8b86-4847-b8af-ab372f36d1fd
DateParserRU: suport 'до'

Add 'до' both as a Date.MOD_BEFORE modifier token
and as a span2 token (i.e., stop end for a Date.MOD_SPAN).

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@23174 4ae1f11a-8b86-4847-b8af-ab372f36d1fd
Revert "6926: display genitive/nominative months as needed"

Sorry, my mistake. Committed from the wrong branch!

This reverts commit 0e82e0077ac1cad122f4584d62b54855ca02e4d8.

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@23175 4ae1f11a-8b86-4847-b8af-ab372f36d1fd