Bug 317417 [Model] StackElement should be abstract and interface
StackElement is only a marker for improved containment hierarchies. We should at least make it abstract but also as an interface like WindowElement, ...
bundles/org.eclipse.e4.ui.model.workbench.edit/src/org/eclipse/e4/ui/model/application/ui/advanced/provider/PerspectiveStackItemProvider.java
(((ComposeableAdapterFactory)adapterFactory).getRootAdapterFactory(),
				 getResourceLocator(),
				 getString("_UI_ElementContainer_selectedElement_feature"), 
				 getString("_UI_PropertyDescriptor_description", "_UI_ElementContainer_selectedElement_feature", "_UI_ElementContainer_type"), 
				 UiPackageImpl.Literals.ELEMENT_CONTAINER__SELECTED_ELEMENT,
				 true,
				 false,
				 true,
				 null,
				 null,
				 null));
	}
	/**
	 * This specifies how to implement {@link #getChildren} and is used to deduce an appropriate feature for an
	 * {@link org.eclipse.emf.edit.command.AddCommand}, {@link org.eclipse.emf.edit.command.RemoveCommand} or
	 * {@link org.eclipse.emf.edit.command.MoveCommand} in {@link #createCommand}.
	 * <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * @generated
	 */
	@Override
	public Collection<? extends EStructuralFeature> getChildrenFeatures(Object object) {
		if (childrenFeatures == null) {
			super.getChildrenFeatures(object);
			childrenFeatures.add(UiPackageImpl.Literals.ELEMENT_CONTAINER__CHILDREN);
		}
		return childrenFeatures;
	}
	/**
	 * <!-- begin-user-doc -->