Handle window buttons, esp. close.

It would be nice if you could intercept and handle the window buttons so you can do things like confirm, show save progress while closing, or save window position - this might be something I'm missing (something with `onunload` in the view?) in which case this might just be a doc bug.

If this isn't currently possible, the simplest option might be to forward them into the JS like host calls, otherwise it looks like it might be a API breaking change in `WebViewBuilder` to add more fields.