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
}
    /*
     * (non-Javadoc) Method declared on Dialog.
     */
    protected void createButtonsForButtonBar(Composite parent) {
        buttons = new Button[buttonLabels.length];
        for (int i = 0; i < buttonLabels.length; i++) {
            String label = buttonLabels[i];
            Button button = createButton(parent, i, label,
                    defaultButtonIndex == i);
            buttons[i] = button;
        }
    }
    /**
     * Creates and returns the contents of an area of the dialog which appears
     * below the message and above the button bar.
     * <p>
     * The default implementation of this framework method returns
     * <code>null</code>. Subclasses may override.
     * </p>
     * 
     * @param parent
     *            parent composite to contain the custom area
     * @return the custom area control, or <code>null</code>
     */
    protected Control createCustomArea(Composite parent) {
        return null;
    }
    /**
     * This implementation of the <code>Dialog</code> framework method creates