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
* and lays out a composite and calls <code>createMessageArea</code> and
     * <code>createCustomArea</code> to populate it. Subclasses should
     * override <code>createCustomArea</code> to add contents below the
     * message.
     */
    protected Control createDialogArea(Composite parent) {
        
        createMessageArea(parent);
        
        Composite composite = new Composite(parent, SWT.NONE);
        GridLayout layout = new GridLayout();
        layout.marginHeight = 0;
        layout.marginWidth = 0;
        composite.setLayout(layout);
        GridData data = new GridData(GridData.FILL_BOTH);
        data.horizontalSpan = 2;
        composite.setLayoutData(data);
        
        customArea = createCustomArea(composite);
        
        if (customArea == null) {
			customArea = new Label(composite, SWT.NULL);
		}
        return composite;
    }
    /**
     * Gets a button in this dialog's button bar.
     * 
     * @param index
     *            the index of the button in the dialog's button bar