STYLE: Make protected `Object::SubjectImplementation` members private
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`.

InsightSoftwareConsortium/ITK,richardbeare/ITK,BRAINSia/ITK,InsightSoftwareConsortium/ITK,BRAINSia/ITK,Kitware/ITK,hjmjohnson/ITK,Kitware/ITK,Kitware/ITK,BRAINSia/ITK,BRAINSia/ITK,richardbeare/ITK,richardbeare/ITK,hjmjohnson/ITK,hjmjohnson/ITK,Kitware/ITK,InsightSoftwareConsortium/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,richardbeare/ITK,hjmjohnson/ITK,InsightSoftwareConsortium/ITK,richardbeare/ITK,InsightSoftwareConsortium/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,richardbeare/ITK,BRAINSia/ITK,Kitware/ITK,Kitware/ITK,BRAINSia/ITK,BRAINSia/ITK,richardbeare/ITK,hjmjohnson/ITK,hjmjohnson/ITK