Add translation for media entities

In preparation of this ticket i found this function in src/TranslationManager.php:

  public function getTranslatableFields($entity) {
    switch ($entity->getEntityTypeId()) {
      case 'node':
        return [
          'title'       => ['value'],
          'body'        => ['value', 'summary'],
          'field_image' => ['alt'],
        ];

      case 'comment':
        return [
          'subject'      => ['value'],
          'comment_body' => ['value'],
        ];

      case 'taxonomy_term':
        return ['name' => ['value']];

      default:
        // Type not implemented.
        return [];
    }
  }

You hardcoded the translatable fields. But what to do with all the other fields? And you can not presume that these fields really exists. This should be replaced by a field iterator per entity.

Added support for media & other type translatable entities in versions 1.1.x-dev and 1.1.0.