Portal Type as Classes: Interaction Workflows methods could be improperly generated because of 58d4ab8e.

1. Add Interaction on _setSourceReference() to `Document Component`.
2. generatePortalTypeClass() is called recursively and an inner `Document
   Component` Portal Type class is created without any accessor holder.
=> As __bases__ is (klass, GetAcquireLocalRolesMixIn) since 58d4ab8e but loadClass()
   was not updated to reflect this, Portal Type Workflow Methods were generated on
   the inner Portal Type Class where _setSourceReference() is not available and
   registerWorkflowMethod() creates a WorkflowMethod calling Base._doNothing.