Also, make sure that the parent is notified when they change.

Fixes #2298

Also, add more dartdoc to widgets.dart.

It's safe to remove the unneeded `void`s from setters since the blocking issues in the
`always_declare_return_types` lint have been fixed (https://github.com/dart-lang/linter/).  We can also safely flip the bit on  `avoid_return_types_on_setters`.

* LayoutBuilder Widget