Change Jenkins to use a single consistent data-binding field.

See also: https://github.com/stapler/stapler/pull/39

This commit includes several changes to accommodate the aforementioned change to Stapler (moving away from 'stapler-class'), as well as to make Jenkins consistently use this same JSON field (moving away from 'kind').


NOTE: Stapler will continue to accept 'stapler-class' for backwards compatibility, and similarly this will continue to pass 'stapler-class' for plugins that have keyed off of it.
Sadly "kind" is too generic for a grep to be useful.

A search for "kind" (quoted) returned zero hits after this change to the core.

If you know how I might search (with quotes) in GitHub's search box, I'd be happy to check more broadly, but there are hundreds of pages hits without the quotes, mostly erroneous.