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
* <!-- end-user-doc -->
	 * @generated
	 * @ordered
	 */
	public static final int WINDOW_ELEMENT__CUR_SHARED_REF = UiPackageImpl.UI_ELEMENT__CUR_SHARED_REF;
	/**
	 * The feature id for the '<em><b>Visible When</b></em>' containment reference.
	 * <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * @generated
	 * @ordered
	 */
	public static final int WINDOW_ELEMENT__VISIBLE_WHEN = UiPackageImpl.UI_ELEMENT__VISIBLE_WHEN;
	/**
	 * The number of structural features of the '<em>Window Element</em>' class.
	 * <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * @generated
	 * @ordered
	 */
	public static final int WINDOW_ELEMENT_FEATURE_COUNT = UiPackageImpl.UI_ELEMENT_FEATURE_COUNT + 0;
	/**
	 * The meta object id for the '{@link org.eclipse.e4.ui.model.application.ui.basic.impl.TrimBarImpl <em>Trim Bar</em>}' class.
	 * <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * @see org.eclipse.e4.ui.model.application.ui.basic.impl.TrimBarImpl
	 * @see org.eclipse.e4.ui.model.application.ui.basic.impl.BasicPackageImpl#getTrimBar()
	 * @generated
	 */
	public static final int TRIM_BAR = 9;