Add plone.app.iterate support for Dexterity to core via p.a.stagingbehavior

Since plone.app.iterate is in core, and plone.app.contenttypes is the
new default, we should support plone.app.iterate for Dexterity in core.
Unfortunately. the plone.app.stagingbehavior tests don't currently pass
in coredev so I'm just going to put this in a pull request.