N°6414 Move existing AbstractSimpleField::Validate impl to custom validators

- LinkedSetField
- SelectObjectField
- MultipleChoicesField (warning this hierarchy contains non multiple value fields like SelectField !)

Also change AbstractValidator::Validate signature : now we are returning an array of error messages, so that we can return multiple ones

sources/Form/Field/MultipleSelectField.php

@@ -28,5 +28,4 @@ class MultipleSelectField extends SelectField
 {
 	/** @inheritDoc */
 	const DEFAULT_MULTIPLE_VALUES_ENABLED = true;
-
 }