shiny/widget/flex: new package for flex layout

This CL contains a new package, with a Node implementation that will lay
out its children following the CSS flexible layout algorithm.

The algorithm will follow in a subsequent CL.

Change-Id: I9c1af5f9b4b9c6346722b98c48eca521d7c4563d
Reviewed-on: https://go-review.googlesource.com/22683
Reviewed-by: Nigel Tao <nigeltao@golang.org>
2 files changed