Bug 273330 [JFace] Add API to MessageDialog to enable clients to use SWT.SHEET
A new style constant SWT.SHEET was added recently. Without adding new API in JFace, clients who are currently using the convenience methods MessageDialog.openWarning/Error/Information/Question/Confirmation would not be able to open these dialogs as sheets.
bundles/org.eclipse.jface/src/org/eclipse/jface/dialogs/MessageDialog.java
*/
    public final static int WARNING = 4;
    /**
     * Labels for buttons in the button bar (localized strings).
     */
    private String[] buttonLabels;
    /**
     * The buttons. Parallels <code>buttonLabels</code>.
     */
    private Button[] buttons;
    /**
     * Index into <code>buttonLabels</code> of the default button.
     */
    private int defaultButtonIndex;
    /**
     * Dialog title (a localized string).
     */
    private String title;
    /**
     * Dialog title image.
     */
    private Image titleImage;
    /**
     * Image, or <code>null</code> if none.
     */
    private Image image = null;
    /**
     * The custom dialog area.
     */
    private Control customArea;