Sustainable exports

[sneridagh]

If we want to use plone.exportimport not just for "snapshots"/migrations but also for maintaining example content, we need to fix these:

• It overrides always modified and created which are not needed/relevant, maybe for a migration, but not for a snapshot/example content.
• The exportimport.versions field contain an object that changes the UID always... probably to the "previous" version of the last export...
• It also exports fields with no value to null. In Plone RESTAPI idiom this is not needed, since in import all non existent fields should remain empty, not set.
• It exports parents and items this are not required, nor needed by the import.

Maybe we should differentiate both use cases (snapshots/migration and example content)?