AppBar with a custom bottom widget

## See also: - The “Components-Tabs” section of the Material Design specification: [https://material.io/guidelines/components/tabs.html](https://material.io/guidelines/components/tabs.html)- The source code in [https://github.com/flutter/flutter/blob/master/examples/catalog/lib/app_bar_bottom.dart](examples/catalog/lib/app_bar_bottom.dart).Credits: This article was initially posted in Flutter.io.