allow class level trigger call on Event subclasses

e.g. UserEvent::trigger('event', 'class');
should result in a UserEvent to be passed to the handlers instead of
a base\Event.

framework/yii/base/Event.php

@@ -157,7 +157,7 @@ class Event extends Object
 			return;
 		}
 		if ($event === null) {
-			$event = new self;
+			$event = new static;
 		}
 		$event->handled = false;
 		$event->name = $name;