shiny/widget: split off node and theme packages.

This de-clutters the top-level widget package, and focuses its package godoc
more for those using widgets and less for those implementing custom widgets.
The former should be the more common case.

Change-Id: I7e955a1b0aa59d05128fef83903f2d6cc138be68
Reviewed-on: https://go-review.googlesource.com/22934
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
10 files changed