[Doing-301]Adjust the relationship between XRuntime and XViewController, and implement the main part of lib-mode-supporting.

1. Add XRootViewController which inherits from XViewController,
XRuntime holds a weak reference to XRootViewController and
XRootViewController is responsible for creating an instance of XRuntime.
2. Update the startApp: and  closeApp: correspondingly.
[Doing-301]Update the logic of isDefaultAppWebView and add some new methods which can be used to find top view controller.
Update templates app delegate to create XRootViewController which is used as the entry point to xFace.
Remove unneeded headers
[Doing-301]Sync p4 change 18171:Observe client notification and fire client event
Update XRootViewController to define runtime as a private property.