shiny/widget: have NewContainer functions take children as args.

This can simplify building the widget tree, as demonstrated by the
example/layout program.

Also add the widget.WithLayoutData function to facilitate this.

Change-Id: Ia41555ab4d68f0f1057698d637115a80e5d4df3c
Reviewed-on: https://go-review.googlesource.com/23530
Reviewed-by: David Crawshaw <crawshaw@golang.org>
5 files changed