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
public void notifyChanged(Notification notification) {
		updateChildren(notification);
		switch (notification.getFeatureID(MWindow.class)) {
			case BasicPackageImpl.WINDOW__LABEL:
			case BasicPackageImpl.WINDOW__ICON_URI:
			case BasicPackageImpl.WINDOW__TOOLTIP:
			case BasicPackageImpl.WINDOW__CONTEXT:
			case BasicPackageImpl.WINDOW__VARIABLES:
			case BasicPackageImpl.WINDOW__BINDING_CONTEXTS:
			case BasicPackageImpl.WINDOW__X:
			case BasicPackageImpl.WINDOW__Y:
			case BasicPackageImpl.WINDOW__WIDTH:
			case BasicPackageImpl.WINDOW__HEIGHT:
				fireNotifyChanged(new ViewerNotification(notification, notification.getNotifier(), false, true));
				return;
			case BasicPackageImpl.WINDOW__PROPERTIES:
			case BasicPackageImpl.WINDOW__HANDLERS:
			case BasicPackageImpl.WINDOW__MAIN_MENU:
			case BasicPackageImpl.WINDOW__WINDOWS:
			case BasicPackageImpl.WINDOW__SHARED_ELEMENTS:
				fireNotifyChanged(new ViewerNotification(notification, notification.getNotifier(), true, false));
				return;
		}
		super.notifyChanged(notification);
	}
	/**
	 * 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 -->