Create content for each tab.

1. Create a TabController

For tabs to work, you need to keep the selected tab and content
sections in sync.
This is the job of the TabController.

Either create a TabController manually,
or automatically by using a DefaultTabController widget.

Using DefaultTabController is the simplest option, since it
creates a TabController and makes it available to all descendant widgets.

2. Create the tabs

When a tab is selected, it needs to display content.
You can create tabs using the TabBar widget.
In this example, create a TabBar with three
Tab widgets and place it within an AppBar.

By default, the TabBar looks up the widget tree for the nearest
DefaultTabController. If you’re manually creating a TabController,
pass it to the TabBar.

3. Create content for each tab