remove regular Extension, have SimpleExtension take its place, optimise the event system based on SimpleExtension methods

contrib/site_description/main.php

@@ -9,7 +9,7 @@
  *  This extension sets the "description" meta tag in the header
  *  of pages so that search engines can pick it up
  */
-class SiteDescription extends SimpleExtension {
+class SiteDescription extends Extension {
 	public function onPageRequest(PageRequestEvent $event) {
 		global $config, $page;
 		if(strlen($config->get_string("site_description")) > 0) {