Migrate SubFormAdapter from z3c.from

[thomasmassmann]

Continuing from the PR in https://github.com/collective/collective.z3cform.datagridfield/pull/116:

So you are saying we need to keep using a version from 5 years ago because of the removed ObjectSubForm and SubFormAdapter? If it is required in Plone, why not add it to plone.z3cform instead and use the latest stable release of z3c.form?

Yes, its that worse. Nobody ever looked into it. It is probably time. But my personal z3c.form Zen-level was never enough for this. Putting it in plone.z3cform is a good idea. Would you give it a try?

• [x] Add implementation for ObjectSubForm and SubFormAdapter from z3c.form
• [x] Check compatibility with z3c.form 3.3.0 to 3.5 and 4.x (ObjectSubForm was removed in 3.3.0, but with 3.6.0 re-released from 3.2.11 — very confusing)

[petschki]

removed adapters got backported here https://github.com/plone/plone.z3cform/pull/20/commits/0cbef33ce26f710d3b2e21b139044d472c31ab02