Reimplementation of constraintypes as z3c.form.

All old tests stay valid.
In case you wonder why:
The z3c.form based browser views have been created for
plone.app.contenttypes, at that time I was unaware that there is
a newer implementation of constraintypes. Only after I finished it
and looked around where to integrate it, I found out about this branch.
Since using a form library for a form makes sense, I decided
to reimplement it. From what I can tell all features survived.
Changenotes, provides marker