More lang variant stuff:

• More parser tests, also added 'variant' option for testing
• Add global $wgDefaultLanguageVariant, can be used to set a default fallback variant
• Restructured some of the parser code in LanguageConverter, fix flag processing and add some comments to despookify the old zh code :)
• Cleanup of some old hacks in LanguageSr and Kk (do not use the global title object, but the one passed to the parser)