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/GenericStackItemProvider.java
extends ElementContainerItemProvider
	implements
		IEditingDomainItemProvider,
		IStructuredItemContentProvider,
		ITreeItemContentProvider,
		IItemLabelProvider,
		IItemPropertySource {