[form] fix #719285, due to multiple calls to restore_previous_post, by proper refactorings * move __init__ code from FieldsForm to Form. Must behaviour here should actually be in the Form base class * avoid buggy duplicated call to restore_previous_post * move some code that was in the form renderer to the form'__init__ method (__redirectpath & __form_id hidden input handling)) * 'formvid' should now be specified on form selection, not on form rendering

"""cube-specific forms/views/actions/components

:organization: Logilab
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:license: GNU Lesser General Public License, v2.1 - http://www.gnu.org/licenses
"""