Public API OC6 - introduce lib/private

move the private namespace OC into lib/private - OCP will stay in lib/public

I had to resubmit this issue because the branch appframework-master has been merged



As discussed in #4863 (comment) we clearly separate public namespace from private namespace.

lib/public - OCP
lib/private - OC

@karlitschek @blizzz @bartv2 @bantu @icewind1991 @kabum THX