Rename gettext to cwgettext, use stdlib gettext
cubicweb/gettext.py was a copy of Python 2.6's gettext.py with the
context patch from bug https://bugs.python.org/issue2504
Instead of keeping that full copy, just inherit from
gettext.GNUTranslations and add the context-related stuff.
rql('SET X name "BoundaryConstraint" '
'WHERE X is CWConstraintType, X name "BoundConstraint"')