(I)AnnotationValueService:

 - List<AnnotationEntry> getValues(IModelElement element) --> AnnotationEntry getAnnotationEntry(IModelElement element);
 - The semantics of the method is change to return exactly the AnnotationEntry that contains the AnnotationValueProvider-to-IAnnotatedSpecification mapping for the given model element. In the old semantics, a list of all annotation entries for model elements that had the same type as the given model element have been returned.

AnnotationEntry:
 - add isEmpty()

Consequences:
 - Fix update bug of GenericAnnotationView when deleting model elements (stale rows used to stay in the view)
 - Performance improvement

TODO: The GenericAnnotationView is still very slow during the deletion of large sub-models (and probably also during addition e.g., because of c'n'p of large sub-models) 
refs 1841