Defer initialisation of the special page list, allow autoloading of the SpecialPage...

Defer initialisation of the special page list, allow autoloading of the SpecialPage class. SpecialPage.php is not loaded for ordinary page views. Implemented some interfaces which allow special pages to do the same thing, although backwards compatibility is maintained. Old special page extensions will load SpecialPage during initialisation, so all extensions will have to be ported to get the full performance benefit.