Update image widget, create icon widget,layout widget

flutter_layout.md

@@ -0,0 +1,65 @@
+Add the layout widget to the page
+
+This provides a default banner, background color, and has API for adding drawers, snack bars, and bottom sheets.
+Then you can add the Center widget directly to the body property for the page.This is mostly using in MaterialApp.This code is show how to do it.
+
+class MyApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'Flutter layout demo',
+      home: Scaffold(
+        appBar: AppBar(
+          title: Text('Flutter layout demo'),
+        ),
+        body: Center(
+          child: Text('Hello World'),
+        ),
+      ),
+    );
+  }
+}
+
+For a non-Material app, you can add it follow this code.By default a non-Material app doesn’t include an AppBar, title, or background color. If you want these features in a non-Material app, you have to build them yourself.
+This app changes the background color to white and the text to dark grey to mimic a Material app.
+
+class MyApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      decoration: BoxDecoration(color: Colors.white),
+      child: Center(
+        child: Text(
+          'Hello World',
+          textDirection: TextDirection.ltr,
+          style: TextStyle(
+            fontSize: 32,
+            color: Colors.black87,
+          ),
+        ),
+      ),
+    );
+  }
+}
+
+create an image widget.
+
+Image.asset(
+  'images/lake.jpg',
+  fit: BoxFit.cover,
+),
+
+create an icon widget.
+
+Icon(
+  Icons.star,
+  color: Colors.red[100],
+),
+
+Add the visible widget to the layout.
+A child property if they take a single child – for example, Center or Container
+A children property if they take a list of widgets – for example, Row, Column, ListView, or Stack.
+
+Center(
+  child: Text('Hello World'),
+),