Replace _save_values and _changed_values property by method call

Those methods are often subject to raising exception during development but those exceptions are often hidden by the property decorator which interpret them as an AttributeError like if _save_values or _changed_values where missing.
Ex: https://discuss.tryton.org/t/attributeerror-sale-sale-model-has-no-attribute-save-values/7370

It will be easier to develop and debug if the real exception was raised.