See https://www.google.com/design/spec/style/writing.html#

Gallery Tweaks

* left -> leading (Removes an LTR bias)
* center -> title (Widget was actually centered)
* right -> actions (Removes an LTR bias, asymmetric with leading)

Fixes #2348

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

Fixes #1382

The example in the spec with multiple actions is an anti-example.

Fixes #1876