Add support for IAnnotatedSpecifications for which multiple instances can exist:

- Extend IAnnotationValueProvider with getter/setter methods to access individual instances of a multi-instance IAnnotatedSpecification

- The storage of the instances is based on an EMap<String, V> in the corresponding IAnnotatedSpecification.

- See MultiInstanceAnnotationValueProviderBase for documentation how to create the corresponding meta-model for the annotation types, and of course use it as a basis for specific multi-instance value providers.

- Merge SingleAttributeValueProviderBase and SingleEDataValueProviderBase to SingleEStructuralFeatureValueProviderBase, and generalize it to work on EStructuralFeatures. This is required to support both EAttributes and "contained" EReferences as annotation storage in the meta-model.

- With the above change, cleanup and extend the instantiation of annotation attributes / contained references to EClasses.

- Extend the GenericAnnotationView to enable instantiation of multi-instance annoations.
refs 1841