silveright application启动过程：
1. The user requests the HTML entry page in the browser.
2. The browser loads the Silverlight plug-in. It then downloads the XAP file that
contains your application.
3. The Silverlight plug-in reads the AppManifest.xml file from the XAP to find out
what assemblies your application uses. It creates the Silverlight runtime
environment and then loads your application assembly (along with any
dependent assemblies).
4. The Silverlight plug-in creates an instance of your custom application class
(which is defined in the App.xaml and App.xaml.cs files).
5. The default constructor of the application class raises the Startup event.
6. Your application handles the Startup event and creates the root visual object
for your application.