Fixes BC break with existing skeleton

- Mvc\TranslatorServiceFactory was missing the "implements FactoryInterface"
  declaration, making it fail as a valid factory.
- Circular dependency was occurring, as changes in TranslatorServiceFactory
  meant it was looking for existence of an alias already defined in the
  skeleton, and pointing back at the same service. Added a
  'Zend\I18n\Translator\TranslatorInterface' service, pointing to the I18n
  TranslatorServiceFactory, and now check for that in the MVC
  TranslatorServiceFactory. This fixes the primary BC issue.
- Updated View\HelperPluginManager to look for (and conditionally fetch) the new
  'Zend\I18n\Translator\TranslatorInterface' service, instead of the
  'translator' service. This will preserve the intent. Developers may alias
  'translator' to 'Zend\I18n\Translator\TranslatorInterface' if desired.
Remove service from ServiceListenerFactory

- Point of original PR that broke BC was to remove the requirement of
  ext-intl by default. Adding the TranslatorInterface as a service by
  default breaks this. Removed, and we can now document it.
Added note to README

- Covers #5406, and the changes also present in this PR.
More BC fixes

- Check for each of MvcTranslator, Translator, and TranslatorInterface
  in order to perform injections.
- Since "Translator" is usually an alias of "MvcTranslator", check this last.
Retain BC in all situations

- If no `TranslatorInterface` service is found, check for "translator"
  configuration, and use it to create a
  `Zend\I18n\Translator\Translator` instance, per original behavior.
- **Always** wrap a created translator instance in a
  `Zend\Mvc\I18n\Translator` instance.
Remove Validator\TranslatorInterface implementation

- No longer needed, as the MVC translator service factory now wraps a
  given translator in an Mvc\I18n\Translator instance.
- Also, CS fixes
Merge branch 'weierophinney-hotfix/translator-service' into develop