shiny/widget: add FlowLayoutData and Node.LayoutData.

Also fix a bug in FlowClass.Measure.

Change-Id: Ie2ff19b89dbdbbc022e4ef6193f175864c30feca
Reviewed-on: https://go-review.googlesource.com/22078
Reviewed-by: David Crawshaw <crawshaw@golang.org>
3 files changed