Complete PrtCnv Qt6 migration so CoPrtCnv/PrtCnvLib/PrtCnv/ddsview build

Restore dropped m_logger members, QMutex->QRecursiveMutex, splitView->split, error->errorOccurred (migration-only, verified vs legacy).

qt6-after/qt6-migrated-legatus/TetraIF/PrtCnv/PrtCnvThreadStarter.cpp

@@ -33,6 +33,7 @@ PrtCnvThreadStarter::PrtCnvThreadStarter(const PrtCnv::PrtCnvApplication::E_Thre
   , m_Application(a_mainApp)
   , m_Id(a_id)
   , m_settings(new NGL::Settings(configFileName))
+  , m_logger(getLogger("PrtCnvThreadStarter"))
 {
   DEBUG_LOG(m_logger) << "Using Config file '" << configFileName << "'";
 }