设置里，标题栏有后退按钮 
在win平板，可以有后退键，手机也有
pc可以在标题栏
在OnLaunched
//最后
Windows.UI.Core.SystemNavigationManager.GetForCurrentView().BackRequested += BackRequested; Windows.UI.Core.SystemNavigationManager.GetForCurrentView().AppViewBackButtonVisibility = Windows.UI.Core.AppViewBackButtonVisibility.Visible;
BackRequested后退方法