Corrected the microsoft-ajax PageLoadingEventArgs class.

Note that although the documentation sometimes incorrectly refers to get_panelsDeleted()  - e.g. on http://msdn.microsoft.com/en-us/library/bb397684%28v=vs.100%29.aspx - the actual function is get_panelsDeleting() - see http://msdn.microsoft.com/en-us/library/bb310960%28v=vs.100%29.aspx