[FIX] fields: in to_column(), returning self.column is generally not correct

Consider the following example:

    class Foo(models.Model):
        _name = 'foo'
        _columns = {
            'state': fields.selection([('a', 'A')]),
        }

    class Bar(models.Model):
        _inherit = 'foo'
        state = fields.Selection(selection_add=[('b', 'B')])

The attribute 'column' of the field does not have the full selection list,
therefore the column object cannot not be reused, even a copy of it.  The
solution is to systematically recreate the column from the field's final
specification, except for function fields that have no sensible way for being
recreated.

This fixes issues #2088 and #2731.