STYLE: Make protected Object::SubjectImplementation members private

The internal helper class `SubjectImplementation` is not designed as a base
class, so in general, its members should be either `private` or `public`.

Modules/Core/Common/src/itkObject.cxx

@@ -88,7 +88,7 @@ class ITKCommon_HIDDEN Object::SubjectImplementation
 
   bool m_ListModified{ false };
 
-protected:
+private:
   // RAII of ListModified state to ensure exception safety
   struct SaveRestoreListModified
   {
@@ -111,7 +111,6 @@ class ITKCommon_HIDDEN Object::SubjectImplementation
   void
   InvokeEventRecursion(const EventObject & event, TObject * self, std::list<Observer>::reverse_iterator & i);
 
-private:
   std::list<Observer> m_Observers;
   unsigned long       m_Count;
 };