LocaleListener should also handle subrequests

right now subrequests are ignored. this means that subrequests will run in the default locale. furthermore there can now be different output for subrequests and for ESI requests which can lead to inconsistency. as a result imho we cannot skip subrequests. however for the locale change event it makes sense to limit this to the master request.

in most cases for subrequests people will need to enable the query guesser and do something like ``{% render ... with {'_locale': app.request.locale} %}`` to propagate the request locale.

fix for #56

related https://github.com/symfony/symfony/issues/6932