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/provider/ElementContainerItemProvider.java
* This adds {@link org.eclipse.emf.edit.command.CommandParameter}s describing the children
	 * that can be created under this object.
	 * <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * @generated
	 */
	@Override
	protected void collectNewChildDescriptors(Collection<Object> newChildDescriptors, Object object) {
		super.collectNewChildDescriptors(newChildDescriptors, object);
		newChildDescriptors.add
			(createChildParameter
				(UiPackageImpl.Literals.ELEMENT_CONTAINER__CHILDREN,
				 MApplicationFactory.INSTANCE.createApplication()));
		newChildDescriptors.add
			(createChildParameter
				(UiPackageImpl.Literals.ELEMENT_CONTAINER__CHILDREN,
				 MMenuFactory.INSTANCE.createMenuSeparator()));
		newChildDescriptors.add
			(createChildParameter
				(UiPackageImpl.Literals.ELEMENT_CONTAINER__CHILDREN,
				 MMenuFactory.INSTANCE.createMenu()));
		newChildDescriptors.add
			(createChildParameter
				(UiPackageImpl.Literals.ELEMENT_CONTAINER__CHILDREN,
				 MMenuFactory.INSTANCE.createMenuContribution()));
		newChildDescriptors.add
			(createChildParameter
				(UiPackageImpl.Literals.ELEMENT_CONTAINER__CHILDREN,
				 MMenuFactory.INSTANCE.createPopupMenu()));
		newChildDescriptors.add