Make xwiki.cfg accessible without XWiki instance

Being force to fin a way to access an instance of XWiki class (generally trough XWikiContext) is quite a pain in many cases especially for components. And initializing XWiki instance is totally useless technically when all you want is accessing xwiki.cfg properties.

So the idea is to implement a xwiki.cfg based (with same behavior than old XWikiConfig class) ConfigurationSource.