6957: VCF export/import should support [x-]gender

Added support for exporting X-GENDER
and importing X-GENDER/GENDER fields.

Tested with impex.sh round-trip (see the bug for details)
and via logger observation.

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/branches/maintenance/gramps34@22762 4ae1f11a-8b86-4847-b8af-ab372f36d1fd