Disable fading effect of dialogs

* Upgrade Notes:
  + The animating effect of window component in modal mode is changed. 
    If you'd like the previous effect, you can use the setDefaultActionOnShow() 
    method of window component to set the empty string ("") or use an easier way to
    define a preference name that a deployer can use to configure in WEB-INF/zk.xml.
    For example, 
    <preference>
        <name>org.zkoss.zul.Window.defaultActionOnShow</name>
        <value>moveDown</value>
    </preference>