Improve EReferenceListPropertySectionBase and make list view reusable

* EReferenceListPropertySectionBase
 * Introduce ElementListViewerComposite: Reusable Composite that
   provides a ListViewer & add/remove buttons
 * Usability improvements: enabled state of buttons, auto-select
   next element after "remove" operation
* Introduce IListPropertySection
  * To be implemented by classes that want to embed
    ElementListViewerComposite
  * Provides interface to data model

Issue-Ref: 3470
Issue-Url: https://af3-developer.fortiss.org/issues/3470



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