shiny/widget: remove the Node.ClassData field.

The Node.Class field already has interface type. There's no need for a
separate interface-typed field.

Also unexport the FooClass types such as FlowClass. Users of this
package won't need to refer to them directly.

Change-Id: I08867ca455834c3257220b9f6d178ebb14238013
Reviewed-on: https://go-review.googlesource.com/22112
Reviewed-by: David Crawshaw <crawshaw@golang.org>
4 files changed