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.
Maybe `RequestImpl.getSubmittedForm` could just automatically convert `"$class":"…"` to `"$class":"…","stapler-class":"…"` before returning JSON to other callers? That would avoid the need to actually create both `input` fields in the HTML.

Or `war/src/main/webapp/scripts/hudson-behavior.js#buildFormTree` could do this, though then you are still submitting redundant data in the POST request, which it would be nicer to avoid.