Add menu entry where language can be chosen

[AydinHassan]

Selection should be stored in \PhpSchool\PhpWorkshop\UserState and saved with \PhpSchool\PhpWorkshop\UserStateSerializer

Languages should be configured the actual workshop. So by default we should always have English. If there is only one language we shouldn't display the menu.

Languages could be configured on \PhpSchool\PhpWorkshop\Application with an addLanguage method or something. That method should validate the given language.