back button/title added for window and web scan page

Author: tashi146

lib/constant.dart

@@ -4,3 +4,5 @@ class PackageConstant {
   static const barcodeFileWebPath =
       "assets/packages/simple_barcode_scanner/assets/barcode.html";
 }
+
+String kScanPageTitle = 'Scan barcode/qrcode';

lib/screens/web.dart

@@ -46,6 +46,9 @@ class BarcodeScanner extends StatelessWidget {
         .registerViewFactory(createdViewId, (int viewId) => iframe);
 
     return Scaffold(
+      appBar: AppBar(
+        title: Text(kScanPageTitle),
+      ),
       body: HtmlElementView(
         viewType: createdViewId,
       ),

lib/screens/window.dart

@@ -26,7 +26,9 @@ class WindowBarcodeScanner extends StatelessWidget {
     WebviewController controller = WebviewController();
     bool isPermissionGranted = false;
     return Scaffold(
-      appBar: AppBar(),
+      appBar: AppBar(
+        title: Text(kScanPageTitle),
+      ),
       body: FutureBuilder<bool>(
           future: initPlatformState(
             controller: controller,