Expose webview estimated loading progress value in Flutter

[florisvdg]

It would be great to be able to subscribe to the webview's estimated progress value in Flutter, backed by the estimatedProgress value in iOS and the onProgressChanged callback on Android. With that in data available we can show a sliding progress bar as a widget in Flutter.

On iOS that would be:

webView.addObserver(self, forKeyPath: #keyPath(WKWebView.estimatedProgress), options: .new, context: nil)

On Android:

webView.setWebChromeClient(new WebChromeClient() {
   public void onProgressChanged(WebView view, int progress) {

   }
}

[slightfoot]

relates to #232

[florisvdg]

Fixed in #255.