introduce a default render implementation on the base widget, which
add_media and then call the newly introduced method
_render(form, field, formrenderer), abstract of the base widget class.
This allow a litle code factorisation and avoid to forget the call to
add_media in subclasses.
"""this contains the cube-specific entities' classes
:organization: Logilab
:copyright: 2001-2010 LOGILAB S.A. (Paris, FRANCE), license is LGPL v2.
:contact: http://www.logilab.fr/ -- mailto:contact@logilab.fr
:license: GNU Lesser General Public License, v2.1 - http://www.gnu.org/licenses
"""