Adding NineTileBox widget

doc/examples/widgets/lib/main.dart

@@ -0,0 +1,29 @@
+import 'package:flutter/material.dart';
+import 'package:flame/flame.dart';
+import 'package:dashbook/dashbook.dart';
+
+import 'package:flame/widgets/nine_tile_box.dart';
+
+void main() async {
+  WidgetsFlutterBinding.ensureInitialized();
+
+  final nineTileBoxImage = await Flame.images.load('nine_tile_box.png');
+  final dashbook = Dashbook();
+
+  dashbook.storiesOf('SpritesheetContainer').decorator(CenterDecorator()).add(
+      'default',
+      (ctx) => Container(
+            width: ctx.numberProperty('width', 200),
+            height: ctx.numberProperty('height', 200),
+            child: NineTileBox(
+              image: nineTileBoxImage,
+              tileSize: 16,
+              destTileSize: 50,
+              child: const Center(
+                  child: const Text('Cool label',
+                      style: const TextStyle(color: const Color(0xFFFFFFFF)))),
+            ),
+          ));
+
+  runApp(dashbook);
+}