[IMP] use model._fields instead of model._all_columns to cover all fields

The old-api model._all_columns contains information about model._columns and
inherited columns.  This dictionary is missing new-api computed non-stored
fields, and the new field objects provide a more readable api...

This commit contains the following changes:

 - adapt several methods of BaseModel to use fields instead of columns and
   _all_columns

 - copy all semantic-free attributes of related fields from their source

 - add attribute 'group_operator' on integer and float fields

 - base, base_action_rule, crm, edi, hr, mail, mass_mailing, pad,
   payment_acquirer, share, website, website_crm, website_mail: simply use
   _fields instead of _all_columns

 - base, decimal_precision, website: adapt qweb rendering methods to use fields
   instead of columns