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
public List<IItemPropertyDescriptor> getPropertyDescriptors(Object object) {
		if (itemPropertyDescriptors == null) {
			super.getPropertyDescriptors(object);
			addSelectedElementPropertyDescriptor(object);
		}
		return itemPropertyDescriptors;
	}
	/**
	 * This adds a property descriptor for the Selected Element feature.
	 * <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * @generated
	 */
	protected void addSelectedElementPropertyDescriptor(Object object) {
		itemPropertyDescriptors.add
			(createItemPropertyDescriptor
				(((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