Initial implementation: Provide control to apply value to multiple rows.

- Extend the IAnnotationValueProvider interface to include a method that returns a list of context menu entries with associated actions. The default implementation returns an empty list (implemented in the default interface method). This allows existing annotation value providers to add some default context menu actions by implementing an additional interface while existing AnnotationValueProviders do not need to be adjusted.
- Add an additional descriptor class "AnnotationInstSpec" that allows to uniquely identify the Annotation (& instanceKey) shown as columns in the annotation view. It has been abstracted from the class ColumnHandle.
- Add a method to the AnnotationLabelProvider that allows to query the Annotation for which it has been constructed.
- Extend the GenericAnnotationView to display a context menu for "annotation columns". The content of the context menu is specified by the AnnoationvalueProvider.
- Implement a context menu entry in the interface DefaultAnnotationContextActions that is implemented by the EnergyConsumptionValueProvider.
refs 2361