Split layout2.dart into several files

sky/examples/stocks2/lib/stock_menu.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'package:sky/framework/fn2.dart';
-import 'package:sky/framework/layout2.dart';
 import 'package:sky/framework/components2/popup_menu.dart';
 import 'package:sky/framework/components2/checkbox.dart';
 import 'package:sky/framework/theme/view_configuration.dart';
 
 class StockMenu extends Component {
   static final Style _style = new Style('''
     position: absolute;
     right: 8px;
     top: ${8 + kStatusBarHeight}px;''');
 
@@ skipping 18 matching lines @@
         items: [
           [new Text('Add stock')],
           [new Text('Remove stock')],
           [new ParentDataNode(new Text('Autorefresh'), _flex1), checkbox],
         ],
         level: 4),
         _style
     );
   }
 }