Provides `IObservable` semantics for `NSNotificationCenter` subscriptions

NSNotificationCenterExtensions.cs

public static class NSNotificationCenterExtensions
{
	public static IObservable<NSNotification> ObserveNotification(this NSNotificationCenter notificationCenter, NSString notificationKey) =>
	  Observable.Create<NSNotification>(obs =>
	  {
		  var nsObserver = notificationCenter.AddObserver(notificationKey, obs.OnNext);
		  return Disposable.Create(() => notificationCenter.RemoveObserver(nsObserver));
	  });
}