Fix AttributeError in orderObjects after deleting an object and restarting Zope

[wesleybl]

Backport of #50 to plone.folder 4.x

[mister-roboto]

@wesleybl 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!

[mauritsvanrees]

On this maintenance branch, please keep the _set_order method, even though we won't call it anymore. Someone may have subclassed this and relies on the existence of this method.

Adding a warnings.warn within the method would be nice.
Possibly even add a fix:

if not isinstance(value, PersistentList):
    value = PersistentList(value)

[mauritsvanrees]

Also, I see you started another branch with plone.meta to tackle the QA failures. Some more changes are needed there. I have pushed an extra commit to an own branch on top of yours:
https://github.com/plone/plone.folder/tree/config-with-default-template-2.3.3.dev0-maurits

[Update: two extra commits]