* Introduce a Directionality inherited widget which sets the ambient LTR vs RTL mode (defaulting to null, which means you cannot use directionality-influenced values).
* Make it possible to configure Padding (including Container.padding and Container.margin) using a directionality-agnostic EdgeInsets variant.
* Provide textDirection and verticalDirection controls on Row and Column to make them RTL-aware.
* Introduce a variant of FractionalOffset based on the EdgeInsets variant. Not yet actually used.
* Fix all the tests that depended on Row defaulting to LTR.
* fix examples/layers/widgets/gestures.dart
* use a list of colors to avoid error prone in elseif
* address review comments
* add tests
* keep index to lower complexity
* address review comments
