[Translation] make IdentityTranslater consistent with normal translator

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | n/a

The MessageSelector should not be required in the constructor of the IdentityTranslator because it's also optional for the [Translator](https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Translation/Translator.php#L64).
I also fixed the TranslatorInterface phpdoc and the IdentityTranslator params similar to https://github.com/symfony/symfony/commit/90a20d78357f720493ec8905f41c962766506b83