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
* @return a button in the dialog's button bar
     */
    protected Button getButton(int index) {
        return buttons[index];
    }
    /**
     * Returns the minimum message area width in pixels This determines the
     * minimum width of the dialog.
     * <p>
     * Subclasses may override.
     * </p>
     * 
     * @return the minimum message area width (in pixels)
     */
    protected int getMinimumMessageWidth() {
        return convertHorizontalDLUsToPixels(IDialogConstants.MINIMUM_MESSAGE_AREA_WIDTH);
    }
    /**
     * Handle the shell close. Set the return code to <code>SWT.DEFAULT</code>
     * as there has been no explicit close by the user.
     * 
     * @see org.eclipse.jface.window.Window#handleShellCloseEvent()
     */
    protected void handleShellCloseEvent() {
        
        
        super.handleShellCloseEvent();
        setReturnCode(SWT.DEFAULT);
    }
	/**