Move the DOM event listener function that saves an annotation on
Ctrl+Enter into AnnotationController. This is just for the sake of
keeping all the code in one place (in AnnotationController, not in the
directive's link function) and putting code where it can be tested more
easily.

Note that the controller's save() function was previously called with
scope.$evalAsync() and we're now just calling it synchronously. This
seems to be fine.