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

This patch is a warmup to improving the visuals in the calculator demo.

Related to #4535

* New Material Design calculator example. (Attempt 2 after git fetch upstream).

* Responded to code review.

* Second round of code review.