Change 'klass' for dexterity content types

[alert (Alessandro Pisa)]

@Bogdan

I found an issue.When you del object it runs also handlers which subscribed on remove events, in my case it runs event from p.a.discussion to clean comments.
Otherwise here https://teamspace.upiq.org/trac/wiki/DexterityClassMigration, there it is using parent_folder._delOb(obj.id) to remove object and in this case it doesn't clean comments.

Very good! Did not run in that issue
Thanks for sharing!