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/basic/provider/WindowItemProvider.java
(createItemPropertyDescriptor
				(((ComposeableAdapterFactory)adapterFactory).getRootAdapterFactory(),
				 getResourceLocator(),
				 getString("_UI_Bindings_bindingContexts_feature"), 
				 getString("_UI_PropertyDescriptor_description", "_UI_Bindings_bindingContexts_feature", "_UI_Bindings_type"), 
				 CommandsPackageImpl.Literals.BINDINGS__BINDING_CONTEXTS,
				 true,
				 false,
				 false,
				 ItemPropertyDescriptor.GENERIC_VALUE_IMAGE,
				 null,
				 null));
	}
	/**
	 * This adds a property descriptor for the X feature.
	 * <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * @generated
	 */
	protected void addXPropertyDescriptor(Object object) {
		itemPropertyDescriptors.add
			(createItemPropertyDescriptor
				(((ComposeableAdapterFactory)adapterFactory).getRootAdapterFactory(),
				 getResourceLocator(),
				 getString("_UI_Window_x_feature"), 
				 getString("_UI_PropertyDescriptor_description", "_UI_Window_x_feature", "_UI_Window_type"), 
				 BasicPackageImpl.Literals.WINDOW__X,
				 true,
				 false,
				 false,