Support embedding Edge on Windows through Microsoft's new 'WebView2' backend.

Microsoft has started providing a COM API for their new Chromium based Edge: https://docs.microsoft.com/en-us/microsoft-edge/hosting/webview2.

Supporting this as a backend of Qt WebView would allow customers to provide an HTML5 View inside their apps without having to ship Qt WebEngine. It might also be possible to improve Qt Creator's help view with this.