rkwarddev: some initial support for <!-- i18n: ... --> and i18n_context="..."

git-svn-id: https://svn.code.sf.net/p/rkward/code@4999 7936002b-cd1d-0410-9ce3-d7ebf93b5162
Another chunk of i18n documentation

git-svn-id: https://svn.code.sf.net/p/rkward/code@5000 7936002b-cd1d-0410-9ce3-d7ebf93b5162