* 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.

* started copying stuff into cupertino page route

* extracted from material page route. Ready for testing

* works with button and gesture

* tests and docs

* review notes

* review notes

Move some Cupertino colors to a common file. 

Create a CupertinoTabBar widget to mimic iOS looks