Handle window buttons, esp. close.

[simonbuchan]

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.