Refactor deferrable updates into classes & interfaces, also add helper method for...

Refactor deferrable updates into classes & interfaces, also add helper method for the most common use case:

$wgDeferredUpdateList[] = new HTMLCacheUpdate( $title, "sometable" );

I kept b/c with $wgDeferredUpdateList for now, but seeing as only 3 exts in svn use it (FileSearch, FlaggedRevs and WikiScripts), I'd like to deprecate it pretty soon :)