A way to customize the way WebApp is instantiated

Currently, a developer could specify the class of WebApp to instantiate (by use of web-app-class). However, sometimes the real class depends on the environment, such as using a dynamic proxy to represents a set of interfaces. Thus, it will be better if we could specify a factory rather than the implementation such as

<system-config>
<web-app-factor-class>foo.WebAppFactory</web-app-factor-class>
</system-config>

Notice that we can not use UiFactory since it is too late (WebApp is instantiated before UiFactory – UiFactory is actually instantiated in WebApp's init()).