shiny/widget: add Node methods that forward to Class methods.

Also add a *Node argument to Class.Arity. It's not really necessary, but it
makes it consistent with all of the other Class methods.

Change-Id: I2ff4ff17248ca36b365425a620e11e89e8972bd3
Reviewed-on: https://go-review.googlesource.com/21857
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
3 files changed