new MandatoryValidatorInterface: adds isMandatory() method to ValidatorInterface(). Refs #1753

The new MandatoryValidatorInterface is temporary and deprecated. It
will be merged into ValidatorInterface in the near future.

The AbstractController::prepareView() method checks if a field value
is mandatory and transfer the state to its view.

Only TextInput widget actually displays the "mandatory" CSS class, for
mandatory fields.