Add way to trigger graceful shutdown for Dioxus Desktop apps

For Dioxus Desktop, when using a mode other than `WindowCloseBehaviour::LastWindowExitsApp`, currently there's no way to quit the app gracefully by stopping the event loop, having to resort to non-graceful ways like `std::process::exit(0)`.

It looks like graceful shutdown is already supported internally by the existing`UserWindowEvent::Shutdown`, but there's no public way to emit that event, as `UserWindowEvent` itself is not even public. So if this won't be made public, an alternative way should be provided.