Reimplementation of backported ObjectSubForm and ISubformFactory adapter

[petschki]

this needs plone/plone.z3cform#20

[mister-roboto]

@petschki thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[petschki]

@mauritsvanrees is it ok to create a 1.9.x branch here for Plone 5.2.x buildout.coredev this PR needs the backports in plone.z3cform which only work in Plone 6 so I would bump this version to 2.0.0.dev0

[mauritsvanrees]

Please create a 1.x then.

[petschki]

done

[petschki]

tested here plone/buildout.coredev#782