Manage annotation events effectively

In addition to controlling annotation changes via our create, update, and delete APIs, we also expose a number of events that can be used to intercept annotation changes that originate in the user interface (UI) by clicks from the user.

Event	Use case
`IAnnotation.Focused`	Fires when an annotation is focused.
`IAnnotation.Unfocused`	Fires when an annotation loses focus.
`IAnnotation.Pressed`	Fires when an annotation is pressed.
`IAnnotation.Selected`	Fires when an annotation is selected.
`IAnnotation.Deselected`	Fires when an annotation is deselected.
`IAnnotationManager.AnnotationsChanged`	Fires when annotations change either due to user action or via the API.
`IAnnotationManager.AnnotationsChanging`	Fires when a user starts or ends an interaction with an annotation.
`IAnnotationManager.AnnotationsCreated`	Fires when a new annotation is created.
`IAnnotationManager.AnnotationsDeleted`	Fires when annotations were deleted.
`IAnnotationManager.AnnotationsLoaded`	Fires when annotations are loaded from annotation provider.
`IAnnotationManager.AnnotationsSaving`	Fires before annotations will be saved.
`IAnnotationManager.AnnotationsSaved`	Fires before annotations were saved.
`IAnnotationManager.AnnotationsUpdated`	Fires when annotations were updated.

annotation.Focused += () => {
  Console.WriteLine("Annotation focused");
};

annotation.Unfocused += () => {
  Console.WriteLine("Annotation unfocused");
};

annotation.Pressed += (isSelected, preventDefault) => {
  Console.WriteLine("Annotation pressed");
};

annotation.Selected += () => {
  Console.WriteLine("Annotation selected");
};

annotation.Deselected += () => {
  Console.WriteLine("Annotation deselected");
};

annotationManager.AnnotationsChanged += () => {
  Console.WriteLine("Annotations changed");
};

annotationManager.AnnotationsChanging += (annotations, reason) => {
  if (reason.AnnotationsChaningReason.DrawEnd) {
    Console.WriteLine("Annotations changing");
  }
};

annotationManager.AnnotationsCreated += (annotations) => {
  foreach(var annotation in annotations) {
    Console.WriteLine(annotation.id);
  }
};

annotationManager.AnnotationsDeleted += (annotations) => {
  foreach(var annotation in annotations) {
    Console.WriteLine(annotation.id);
  }
};

annotationManager.AnnotationsLoaded += (annotations) => {
  foreach(var annotation in annotations) {
    Console.WriteLine(annotation.id);
  }
};

annotationManager.AnnotationsSaving += () => {
  Console.WriteLine("Saving annotations");
};

annotationManager.AnnotationsSaved += () => {
  Console.WriteLine("Saved annotations");
};

annotationManager.AnnotationsUpdated += (annotations) => {
  foreach(var annotation in annotations) {
    Console.WriteLine(annotation.id);
  }
};