Restore numbering to navigator items so we can use iterator.

Otherwise the numbering is by priority which is not traversable
(1000, 2000, 3000...).

code/controllers/SilverStripeNavigator.php

@@ -59,7 +59,8 @@ class SilverStripeNavigator extends ViewableData {
 		}
 		ksort($items);
 
-		return new ArrayList($items);
+		// Drop the keys and let the ArrayList handle the numbering, so $First, $Last and others work properly.
+		return new ArrayList(array_values($items));
 	}
 	
 	/**