flutter: Double back to close App

WAR fix #204: Going back leaves the app
Pressing the back button closes the application when you are
in Preferences or other sections, it's annoying.

Known Issues:
* String "Tap back again to leave." is not translated.
* Inability to perceive gestures.

src/ui/flutter_app/lib/l10n/intl_en.arb

@@ -919,5 +919,9 @@
   "atEnd": "At the end of move list.",
   "@atEnd": {
     "description": "At the end of move list."
+  },
+  "tapBackAgainToLeave": "Tap back again to leave.",
+  "@tapBackAgainToLeave": {
+    "description": "Tap back again to leave."
   }
 }

src/ui/flutter_app/lib/l10n/intl_zh.arb

@@ -229,5 +229,6 @@
   "notAIsTurn": "现在不是轮到电脑行棋",
   "aiIsNotThinking": "电脑并非正在思考中",
   "autoReplay": "自动回放",
-  "atEnd": "已经到底了"
+  "atEnd": "已经到底了",
+  "tapBackAgainToLeave": "再次按返回键退出应用"
 }

src/ui/flutter_app/lib/main.dart

@@ -20,6 +20,7 @@ import 'dart:async';
 import 'dart:io';
 
 import 'package:catcher/catcher.dart';
+import 'package:double_back_to_close_app/double_back_to_close_app.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_localizations/flutter_localizations.dart';
@@ -138,13 +139,21 @@ class _SanmillAppState extends State<SanmillApp> {
       ],
       theme: ThemeData(primarySwatch: AppTheme.appPrimaryColor),
       debugShowCheckedModeBanner: false,
-      home: WillPopScope(
-        onWillPop: () async {
-          Audios.disposePool();
-          return true;
-        },
-        child: NavigationHomeScreen(),
+      home: Scaffold(
+        body: DoubleBackToCloseApp(
+          child: NavigationHomeScreen(),
+          snackBar: const SnackBar(
+            content: Text('Tap back again to leave.'),
+          ),
+        ),
       ),
+      /*
+      WillPopScope(
+              onWillPop: () async {
+                Audios.disposePool();
+                return true;
+              },
+      */
     );
   }
 }

src/ui/flutter_app/pubspec.yaml

@@ -24,6 +24,7 @@ dependencies:
   stack_trace: ^1.10.0
   device_info_plus_platform_interface: ^1.0.1
   devicelocale: ^0.4.1
+  double_back_to_close_app: ^2.0.1
 
 dev_dependencies:
   flutter_test: