Plip12253 use z3c.form (rebase)

Seems the pull request to update this package never happened, and there was lots of work on master missing from the branch.

This is a rebase of plip12253-use-z3c.form onto current master. The original plip branch is still there for handy diffing. Seems to work on buildout.coredev.

I also removed the prefix="IEnhancedUserDataSchema" option, since it was entirely useless where specified, should have been part of field.Fields().