* rename DestinationLabel and labels properties to BottomNavigationBarItem and items

* update/fix comments

* grammatical changes

* add myself to AUTHORS

Changed DefaultTextStyle to consider the TextTheme defined in the context for Bottom Nav Bar (#7042)

* Changed DefaultTextStyle to consider the TextTheme defined in the context

* added regression test for bottom nav bar text themes

* added second regression test for fixed navbar

This patch replaces uses of Flexible with Expanded where we're using
FlexFit.tight. We still need to think of a better name for the
FlexFit.loose variant.

Also, improve the docs for Row, Column, Flex, and RenderFlex to be more
problem-oriented and to give a complete account of the layout algorithn.

Fixes #6960
Fixes #5169

The circle's offset represents the position of a navigation button within the
row of buttons.  Previously, this offset had been applied to the entire width
of the bottom bar, not just the button region.

Fixes https://github.com/flutter/flutter/issues/6046

This requires all AnimationController objects to be given a
TickerProvider, a class that can create the Ticker.

It also provides some nice mixins for people who want to have their
State provide a TickerProvider. And a schedulerTickerProvider for those
cases where you just want to see your battery burn.

Also, we now enforce destruction order for elements.

Changed the fixed type to better support the dark theme.

When switching from shifting to fixed, the BottomNavigationBar no
longer displays the old color of the background.