Implement validation action as a class

* src/HAB/Diglib/MultiSchemaValidation/ValidationService.php (addValidationAction):
  Use validation action classes instead partial application.
  (createPartialValidatorApplication): Removed.
* src/HAB/Diglib/MultiSchemaValidation/Action/ActionInterface.php: New
  interface. Validation action.
* src/HAB/Diglib/MultiSchemaValidation/Action/Validate.php: New
  class. Validate candidate.
* src/HAB/Diglib/MultiSchemaValidation/Action/Reject.php: New
  class. Reject candidate.
* src/HAB/Diglib/MultiSchemaValidation/Action/Accept.php: New
  class. Accept candidate.