refactor form field value handling, to get a nicer api and an easier algorithm to get field's value Details: * new .typed_value / .display_value on fields * droped form_field_value on form * .value attribute of field instead of .initial * nicer field's __init__, allowing to give a lambda as value's value

# cube's specific schema
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:organization: Logilab
:copyright: 2001-2009 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
"""