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
.. -*- coding: utf-8 -*-
Securité
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[TRADUISEZ-MOI]