shiny/widget: have Sheet.Paint honor origin and widget rect.

Also split the example/textedit UI into a header and body.

Change-Id: Id95cf599de35ce8a86cf50d9b3ed744b2ab23d6e
Reviewed-on: https://go-review.googlesource.com/25471
Reviewed-by: David Crawshaw <crawshaw@golang.org>
2 files changed