Factorize and cleanup: introduce interfaces and base classes

* Editing support: IMultiValueAnnotationEditingSupport with base class
   MultiValueAnnotationEditingSupportBase
    * currently used in MultiValueAnnotationSelectionEditingSupport
    * integrates cell editor (whose updateData() has been dropped in
      favor of the constructor, i.e. a new cell editor is created each
      time
* Dialogs: IMultiValueAnnotationDialog with base class
  MultiValueAnnotationDialogBase
  * currently used by MultiValueAnnotationSelectionDialog

Issue-Ref: 3267
Issue-Url: https://af3-developer.fortiss.org/issues/3267



Signed-off-by: Simon Barner <barner@fortiss.org>