Improved INotifyCollectionChanged Handling

[Grauenwolf]

This is a carry over from .NET Framework.

If INotifyCollectionChanged.CollectionChanged -> NotifyCollectionChangedEventArgs.NewItems has more than one item, most WPF controls will throw an exception.

The main reason for this original design is that ObservableCollection doesn't have an AddRange method and the controls assum that nothing else will support INotifyCollectionChanged.