shiny/widget: eliminate the Class concept.

There is no longer both Button and buttonClass types. There is now only one
type: Button.

In the shiny/widget/node package, the abstract and concrete types have been
renamed from Class and Node to Node and Embed.

Change-Id: I659edd23f40b8d2b3838e32cec542a5b8d1d5190
Reviewed-on: https://go-review.googlesource.com/22991
Reviewed-by: Nigel Tao <nigeltao@golang.org>
6 files changed