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
*            <li><code>MessageDialog.INFORMATION</code> for a dialog
     *            with an information image</li>
     *            <li><code>MessageDialog.QUESTION </code> for a dialog with a
     *            question image</li>
     *            <li><code>MessageDialog.WARNING</code> for a dialog with a
     *            warning image</li>
     *            </ul>
     * @param dialogButtonLabels
     *            an array of labels for the buttons in the button bar
     * @param defaultIndex
     *            the index in the button label array of the default button
     */
    public MessageDialog(Shell parentShell, String dialogTitle,
            Image dialogTitleImage, String dialogMessage, int dialogImageType,
            String[] dialogButtonLabels, int defaultIndex) {
        super(parentShell);
        this.title = dialogTitle;
        this.titleImage = dialogTitleImage;
        this.message = dialogMessage;
        switch (dialogImageType) {
        case ERROR: {
            this.image = getErrorImage();
            break;
        }
        case INFORMATION: {
            this.image = getInfoImage();
            break;
        }
        case QUESTION: {
            this.image = getQuestionImage();