Fix for regression in variant short-URLs...

Fix for regression in variant short-URLs...
Cleaned up the new URL parsing code a bit, now able to recognize both $wgActionPaths and $wgVariantArticlePath and interpolate those variables.
Moved that bit into WebRequest::interpolateTitle() function which is called after $wgContLang stub object is created, so it can be used to get the variant list.
I'm not totally happy with all this code, but it works. The variant bits generally creep me out.