An optional callback that's called when the TabBar is tapped.
The callback is applied to the index of the tab where the tap occurred.
This callback has no effect on the default handling of taps. It's for applications that want to do a little extra work when a tab is tapped, even if the tap doesn't change the TabController's index. TabBar onTap callbacks should not make changes to the TabController since that would interfere with the default tap handler.