Pass hidden state when opening WebViewScaffold

For some reason WebViewScaffold.hidden property wasn't passed to webviewReference.launch call. Because of that WebView was always visible when shown using scaffold, even when hidden was set to true

lib/src/webview_scaffold.dart

@@ -104,6 +104,7 @@ class _WebviewScaffoldState extends State<WebviewScaffold> {
               withJavascript: widget.withJavascript,
               clearCache: widget.clearCache,
               clearCookies: widget.clearCookies,
+              hidden: widget.hidden,
               enableAppScheme: widget.enableAppScheme,
               userAgent: widget.userAgent,
               rect: _rect,