Adding SimpleGame, EmbeddedGameWidget and animationAsWidget to allow for easy integration of flame with non-game flutter apps, plus docs improved, minor fixes

doc/examples/animation_widget/lib/main.dart

@@ -0,0 +1,62 @@
+import 'package:flame/animation.dart' as animation;
+import 'package:flame/flame.dart';
+import 'package:flame/position.dart';
+import 'package:flutter/material.dart';
+
+void main() => runApp(MyApp());
+
+class MyApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'Animation as a Widget Demo',
+      theme: ThemeData(primarySwatch: Colors.blue),
+      home: MyHomePage(),
+    );
+  }
+}
+
+class MyHomePage extends StatefulWidget {
+  @override
+  _MyHomePageState createState() => _MyHomePageState();
+}
+
+class _MyHomePageState extends State<MyHomePage> {
+  void _clickFab(GlobalKey<ScaffoldState> key) {
+    key.currentState.showSnackBar(new SnackBar(
+      content: new Text('You clicked the FAB!'),
+    ));
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final key = new GlobalKey<ScaffoldState>();
+    return Scaffold(
+      key: key,
+      appBar: AppBar(
+        title: Text('Animation as a Widget Demo'),
+      ),
+      body: Center(
+        child: Column(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: <Widget>[
+            Text('Hi there! This is a regular Flutter app,'),
+            Text('with a complex widget tree and also'),
+            Text('some pretty sprite sheet animations :)'),
+            Flame.util.animationAsWidget(
+                Position(256.0, 256.0),
+                animation.Animation.sequenced('minotaur.png', 19,
+                    textureWidth: 96.0)),
+            Text('Neat, hum?'),
+            Text('Sprites from Elthen\'s amazing work on itch.io:'),
+            Text('https://elthen.itch.io/2d-pixel-art-minotaur-sprites'),
+          ],
+        ),
+      ),
+      floatingActionButton: FloatingActionButton(
+        onPressed: () => _clickFab(key),
+        child: Icon(Icons.add),
+      ),
+    );
+  }
+}