shiny/widget/node: add an OnMouseEvent method.

Change-Id: Ib3e001793e9bcb5c3bfc7577b93914873cd03d30
Reviewed-on: https://go-review.googlesource.com/23071
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
3 files changed