Fix WP8 app exit with back button

CEGLView end() is called from the CCDirector's
end() method, so we should terminate the app then.
The reason for this change is that, the ending of the app can/and probably should be done, at least from the cocos C++ game impl, by calling `end` on the `CCDirector`. Otherwise every app will quit instantly on back key, even if it's not on the main screen :)

The `TerminateApp` is sent when the view has been terminated.