Add support for editing EReferences

 - Extend EStructuralFeatureValueProvider and ComboBoxEditingSupport to handle also this case
 - Reintroduce EStructuralFeatureDescriptor to provide means to specify the scope in which EReferences can be set
     - PROJECT: All models in the current project
     - MODEL: Current model (e.g., ComponentArchitecture)
     - SUB_MODEL: Sub-model below the model element that hosts the annotation managed by the current IAnnotationValueProvider
refs 1841