系统调用app的main函数
main函数调用UIApplicationMain.
UIApplicationMain创建sharedapplication instance, UIApplication默认的instance.
UIApplicationMain读取Info.plist找到主nib文件, 加载nib，把shared applicationinstance 设为nib的owner.
通过nib文件，创建app的独立UIWindows object.
通过nib，实例化了程序的AppDelegate object.
app内部启动结束，application:didFinishLaunchingWith-Options: 被设定成 wAppDelegate instance.
AppDelegate向UIWindowinstance发makeKeyAndVisible消息, app界面展示给用户. app准备好接收用户的操作指令.
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