Change the API so it follows the Flutter BottomNavigationBar

[mg]

• rename selectedIndex to currentIndex
• rename onIndexChanged to onTap

Is there a reason for the API's to be different?

cheers

[mg]

Also:

• BottomNavigationBarItem takes an Icon widget while ExpandingBottomBarItem takes IconData.
• BottomNavigationBarItem uses a parameter named title and accepts a widget for the label while ExpandingBottomBarItem uses a parameter named text and accepts a string.