[Translation] added LoggableTranslator.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #3015, #2435
| License       | MIT
@fabpot actually, the right way to implement it would be to rely on ``decorates`` rather than trying to do it manually. It is exactly what we want here:

```xml
<service id="translator.loggable" class="%translator.loggable.class%" public="false">
    <argument type="service" id="translator.loggable.inner" />
    <argument type="service" id="logger" on-invalid="ignore" />
    <tag name="monolog.logger" channel="translation" />
</service>
```

```php
if ($container->getParameter('kernel.debug')) {
    $container->getDefinition('translator.loggable')->setDecoratedService('translator');
}
```