- 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

- 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

- Provide generic abstract base classes:
  - ValueProviderBase -> for generic annotated specifications
  - SinglePrimitiveAttributeValueProviderBase, for annotated specifications whose storage is implemented by a single primitive attribute (Double, Integer, String, etc.)

- Introduce sub packages for annotation framework (see org.fortiss.tooling.base.ui.annotation -> package.html for documentation)
- Standardize class names

 - add setAnnotationValue(String value, T specification) in addition to setAnnotationValue(V value, T specification) variant
 - This enables to use the String-based AnnotationEditingSupportStandard for non-String annotations.
 - Concrete AnnotationValueProviders must implement this method and convert the String representation of the input to the storage format provided by the respective annotation class (e.g., for Double, just by using the Double(String) constructor)
 - The current implementation  has been tested with String, Double, enum and multiple value String annotations

- TODO:
  - Cleanup of IAnnotationValueProvider
  - Base class hierarchy for common concrete annotation value providers
  - multiple value annotation support in GenericAnnotationView

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fix of failures: was again due to the EMF annoying thing of forcing non-duplicates in function arguments
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