Support WindowEvent stream

[M-Adoo]

Is your feature request related to a problem? Please describe. User need to response to the window resize, close, etc. For example, when the window is closing, user need to release the resource， save the data, etc.

Describe the solution you'd like A WindowEvent stream is needed. User can subscribe to the stream to get the event.

window.events().subscribe(|event| {
    match event {
        WindowEvent::Close => {
          // Cancel the window close.
          event.prevent_default();
        }
        WindowEvent::Resize => {
            // resize the window
        }
    }
});

Describe alternatives you've considered ~~A clear and concise description of any alternative solutions or features you've considered.~~

Additional context

We introduced an AppEvent::WndFocusChanged(WindowId, bool) in #454 which should be removed in favor of this.