Finalize annotation framework overhaul

- IAnnotationValueProvider
  - allowsMultipleValues() -> allowsMultipleAnnotationInstances()
  - createEditingSupport()
     - Remove unused parameter
     - Method is unconditionally used to construct the editing support. Defaults
       are set in base classes implementing IAnnotationValueProvider.
  - remove getFixedValues(): SingleEnumAttributeValueProviderBase() creates the required ComboBoxEditingSupport
  
- IAnnotationValueProvider base classes:
  - ValueProviderBase: Empty implementation of interface
  - SingleAttributeValueProviderBase: IAnnotationValueProvider manages a single attribute of a given specification class. Probably nearly all static annotations will be based on this.
  - SingleEDataTypeAttributeValueProvider: primitive data types (e.g., int, double), and potentially many other data types managed by EMF
  - SingleEnumAttributeValueProvider: Provides ComboBoxEditingSupport based on enum declaration in Ecore meta-model.

- Further fixes:
  - Make combo box in ComboBoxEditionSupport non-editable
  - Prevent NPE in TextEditingSupport.getValue()
  
- Use new base classes in test annotation classes
refs 1841