7158: refactor parsing of "today"

prepare for localization - refactor the parsing of "today" in line with
other token parsing in DateParser, rather than using the hardwired text

Already possible to override per language, but I'll next add
a way to import a single translation of a "today" from the PO.

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@23407 4ae1f11a-8b86-4847-b8af-ab372f36d1fd
7158: localize "today" in DateParser

Should we also support prefixes?

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@23408 4ae1f11a-8b86-4847-b8af-ab372f36d1fd
7158: flag _dateparser.py to string extraction

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@23409 4ae1f11a-8b86-4847-b8af-ab372f36d1fd
7158: POT += "today"

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@23410 4ae1f11a-8b86-4847-b8af-ab372f36d1fd
7158: translate "today" to RU

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@23411 4ae1f11a-8b86-4847-b8af-ab372f36d1fd