Move example fn widgets into sky/framework/components

sky/examples/fn/widgets/box.dart

-part of widgets;
-
-class Box extends Component {
-
-  static Style _style = new Style('''
-    display: flex;
-    flex-direction: column;
-    border-radius: 4px;
-    border: 1px solid gray;
-    margin: 10px;'''
-  );
-
-  static Style _titleStyle = new Style('''
-    flex: 1;
-    text-align: center;
-    font-size: 10px;
-    padding: 8px 8px 4px 8px;'''
-  );
-
-  static Style _contentStyle = new Style('''
-    flex: 1;
-    padding: 4px 8px 8px 8px;'''
-  );
-
-  String title;
-  List<Node> children;
-
-  Box({String key, this.title, this.children }) : super(key: key);
-
-  Node build() {
-    return new Container(
-      style: _style,
-      children: [
-        new Container(
-          key: 'Title',
-          style: _titleStyle,
-          children: [new Text(title)]
-        ),
-        new Container(
-          key: 'Content',
-          style: _contentStyle,
-          children: children
-        ),
-      ]
-    );
-  }
-}