Add an Updated IObservable to CommonReactiveSource.

When you need to react to view-related changes to `CommonReactiveSource` it is not enough to observe the underlying collection from the ViewModel. Due to the buffered nature that `CommonReactiveSource` processes the underlying collection changes, there's a significant lag and you end up handling the old view. Having this `IObservable` lets you track the view-level updates themselves.