Bug 317405 [Model] HandledItem should be abstract
Looks like an oversight the only real types are HandledMenuItem and HandledToolItem.
bundles/org.eclipse.e4.ui.model.workbench.edit/src/org/eclipse/e4/ui/model/application/ui/provider/ElementContainerItemProvider.java
(createChildParameter
				(UiPackageImpl.Literals.ELEMENT_CONTAINER__CHILDREN,
				 MBasicFactory.INSTANCE.createWindow()));
		newChildDescriptors.add
			(createChildParameter
				(UiPackageImpl.Literals.ELEMENT_CONTAINER__CHILDREN,
				 MBasicFactory.INSTANCE.createTrimmedWindow()));
		newChildDescriptors.add
			(createChildParameter
				(UiPackageImpl.Literals.ELEMENT_CONTAINER__CHILDREN,
				 MBasicFactory.INSTANCE.createTrimBar()));
		newChildDescriptors.add
			(createChildParameter
				(UiPackageImpl.Literals.ELEMENT_CONTAINER__CHILDREN,
				 MBasicFactory.INSTANCE.createStackElement()));
		newChildDescriptors.add
			(createChildParameter
				(UiPackageImpl.Literals.ELEMENT_CONTAINER__CHILDREN,
				 MAdvancedFactory.INSTANCE.createPlaceholder()));
		newChildDescriptors.add
			(createChildParameter
				(UiPackageImpl.Literals.ELEMENT_CONTAINER__CHILDREN,
				 MAdvancedFactory.INSTANCE.createPerspective()));
		newChildDescriptors.add
			(createChildParameter
				(UiPackageImpl.Literals.ELEMENT_CONTAINER__CHILDREN,
				 MAdvancedFactory.INSTANCE.createPerspectiveStack()));
	}
}