Bug 317417 [Model] StackElement should be abstract and interface
StackElement is only a marker for improved containment hierarchies. We should at least make it abstract but also as an interface like WindowElement, ...
bundles/org.eclipse.e4.ui.model.workbench/src/org/eclipse/e4/ui/model/application/ui/basic/impl/BasicPackageImpl.java
* @ordered
	 */
	public static final int WINDOW__HEIGHT = UiPackageImpl.ELEMENT_CONTAINER_FEATURE_COUNT + 12;
	/**
	 * The feature id for the '<em><b>Windows</b></em>' containment reference list.
	 * <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * @generated
	 * @ordered
	 */
	public static final int WINDOW__WINDOWS = UiPackageImpl.ELEMENT_CONTAINER_FEATURE_COUNT + 13;
	/**
	 * The feature id for the '<em><b>Shared Elements</b></em>' containment reference list.
	 * <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * @generated
	 * @ordered
	 */
	public static final int WINDOW__SHARED_ELEMENTS = UiPackageImpl.ELEMENT_CONTAINER_FEATURE_COUNT + 14;
	/**
	 * The number of structural features of the '<em>Window</em>' class.
	 * <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * @generated
	 * @ordered
	 */
	public static final int WINDOW_FEATURE_COUNT = UiPackageImpl.ELEMENT_CONTAINER_FEATURE_COUNT + 15;
	/**
	 * The meta object id for the '{@link org.eclipse.e4.ui.model.application.ui.basic.impl.TrimmedWindowImpl <em>Trimmed Window</em>}' class.
	 * <!-- begin-user-doc -->