Refactoring: $smarty, $Config in Context, lang_* in Language class.

[ Previous state ]

* $smarty is already in context but still were initialized in index.php
* $Config are global variables
* L10n code uses lang_* global functions
* The default message fallback language were MESSAGE_FALLBACK_LANG

[ New state ]

* Smarty init code is now in Context::initializeTemplateEngine()
* The configuration is Context::$config.
* A language class contains the L10n logic and lang_* are moved here
* The default message fallback language is Language::FALLBACK

[ Notes ]

* As we don't have a loader for the context, we're keeping temporarily
  the $Config global variable array, so we can refactor static methods
  depending of the configuration to ask a context.

[ Feature description ]

Current
Improving get_server_url() for CLI mode

get_server_url() in CLI mode will return an empty string