flutter: 首页添加“健康游戏忠告”以及版权信息并添加双人对战和机器对战的选项

双人对战和机器对战的功能未完成。

src/ui/flutter/lib/engine/engine.dart

@@ -19,7 +19,7 @@
 
 import 'package:sanmill/mill/position.dart';
 
-enum EngineType { Cloud, Native }
+enum EngineType { humanVsCloud, humanVsAi, humanVsHuman, aiVsAi }
 
 class EngineResponse {
   final String type;

src/ui/flutter/lib/widgets/game_page.dart

@@ -361,7 +361,7 @@ class _GamePageState extends State<GamePage> {
       },
     );
 
-    if (widget.engineType == EngineType.Cloud)
+    if (widget.engineType == EngineType.humanVsCloud)
       Player.shared.increaseWinCloudEngine();
     else
       Player.shared.increaseWinAi();
@@ -448,7 +448,10 @@ class _GamePageState extends State<GamePage> {
               Expanded(child: SizedBox()),
               Hero(tag: 'logo', child: Image.asset('images/logo-mini.png')),
               SizedBox(width: 10),
-              Text(widget.engineType == EngineType.Cloud ? '挑战云主机' : '人机对战',
+              Text(
+                  widget.engineType == EngineType.humanVsCloud
+                      ? '挑战云主机'
+                      : '人机对战',
                   style: titleStyle),
               Expanded(child: SizedBox()),
               IconButton(

src/ui/flutter/lib/widgets/main_menu.dart

@@ -120,10 +120,28 @@ class _MainMenuState extends State<MainMenu> with TickerProviderStateMixin {
           Expanded(child: SizedBox()),
           FlatButton(
             child: Text('人机对战', style: menuItemStyle),
-            onPressed: () => navigateTo(GamePage(EngineType.Native)),
+            onPressed: () => navigateTo(GamePage(EngineType.humanVsAi)),
           ),
           Expanded(child: SizedBox()),
-          Text('Calcitem',
+          FlatButton(
+            child: Text('双人对战', style: menuItemStyle),
+            onPressed: () => navigateTo(GamePage(EngineType.humanVsHuman)),
+          ),
+          Expanded(child: SizedBox()),
+          FlatButton(
+            child: Text('机器对战', style: menuItemStyle),
+            onPressed: () => navigateTo(GamePage(EngineType.aiVsAi)),
+          ),
+          Expanded(child: SizedBox()),
+          Text(
+              '              健康游戏忠告\n'
+              '抵制不良游戏，拒绝盗版游戏。\n'
+              '注意自我保护，谨防受骗上当。\n'
+              '适度游戏益脑，沉迷游戏伤身。\n'
+              '合理安排时间，享受健康生活。',
+              style: TextStyle(color: Colors.black54, fontSize: 16)),
+          Expanded(child: SizedBox()),
+          Text('Copyright © 2019-2020 The Sanmill Authors',
               style: TextStyle(color: Colors.black54, fontSize: 16)),
           Expanded(child: SizedBox()),
         ],