[WIP][FEATURE] Locale detection from URL and HTTP header (Revision 1)

Setting a locale depending on the concrete request had to be done
manually until now. With this change, the HTTP request handler takes
care of the following tasks:

* automatic locale extraction from the Accept-Language HTTP header
* automatic locale extraction from the URL, if given

The URL part requires configured routes, with a custom LocaleRoutePart
handler that is also part of this commit. When a locale is supplied with
the URL, this is also used for building URLs throughout the request.

TODO:

* option for storing the locale in the session (instead of putting it in
  the URL again)
* a view helper that assists in selecting a locale, i.e. does the
  switching stuff and redirects to the previous page
* find out if and how we can use localized URL parts
* (functional and possibly unit) tests
* check if we can limit locale detection from the URL to existing
  locales

Change-Id: Icca47cf9223801e5740c543aaf22bd2a9256e588
Resolves: #35030
Releases: 1.2
Revision-Id: 7f2471f2f232c24f319992e8b7d75a3ad243c129