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.

"""pre 3.6 bw compat"""
# pylint: disable-msg=W0614,W0401
from warnings import warn
warn('moved to cubicweb.mttransforms', DeprecationWarning, stacklevel=2)
from cubicweb.mttransforms import *