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/menu/provider/MenuItemProvider.java
(createChildParameter
				(UiPackageImpl.Literals.ELEMENT_CONTAINER__CHILDREN,
				 MMenuFactory.INSTANCE.createHandledMenuItem()));
		newChildDescriptors.add
			(createChildParameter
				(UiPackageImpl.Literals.ELEMENT_CONTAINER__CHILDREN,
				 MMenuFactory.INSTANCE.createToolBar()));
		newChildDescriptors.add
			(createChildParameter
				(UiPackageImpl.Literals.ELEMENT_CONTAINER__CHILDREN,
				 MMenuFactory.INSTANCE.createToolControl()));
		newChildDescriptors.add
			(createChildParameter
				(UiPackageImpl.Literals.ELEMENT_CONTAINER__CHILDREN,
				 MMenuFactory.INSTANCE.createHandledToolItem()));
		newChildDescriptors.add
			(createChildParameter
				(UiPackageImpl.Literals.ELEMENT_CONTAINER__CHILDREN,
				 MMenuFactory.INSTANCE.createDirectToolItem()));
		newChildDescriptors.add
			(createChildParameter
				(UiPackageImpl.Literals.ELEMENT_CONTAINER__CHILDREN,
				 MMenuFactory.INSTANCE.createToolBarSeparator()));
		newChildDescriptors.add
			(createChildParameter
				(UiPackageImpl.Literals.ELEMENT_CONTAINER__CHILDREN,
				 MMenuFactory.INSTANCE.createRenderedMenu()));
		newChildDescriptors.add
			(createChildParameter
				(UiPackageImpl.Literals.ELEMENT_CONTAINER__CHILDREN,