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#6960Fixes#5169
This API is the front-end part of the work on showing licenses.
Future patches will:
* Provide an API for registering what licenses should be shown here,
which will be used by this feature to shown licenses but could also be
used by custom code for showing licenses (e.g. for people not using
the Material widgets).
* Actually populate this license API from all the licenses we currently
use in the engine, in the framework, and from any pub packages that
are used (directly or indirectly) by the application.
