BUILD_RTI conditional fixes and Qt6 API updates

- cmake/FindRTI.cmake: Add INTERFACE placeholder targets when RTI not found
- cmake/FindRTI.cmake: Force BUILD_RTI=OFF when RTI not installed
- Lts/Common/LtsSession/CMakeLists.txt: Make LtsSessionDataDds conditional on BUILD_RTI
- Lts/Viewer/CMakeLists.txt: Make LtsDataView conditional on BUILD_RTI
- Lts/Plugins/*/CMakeLists.txt: 97 plugin files wrapped in BUILD_RTI conditionals
- cmake/NGLMacros.cmake: Fix NGL_ADD_BIG_RESOURCES macro (removed premature return())
- Multiple Qt6 API fixes: QLinkedList→std::list, QStringRef→QStringView, etc.
- BUILD_RTI_FIXES_SUMMARY.md: Comprehensive documentation of all fixes

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

qt6-after/qt6-migrated-legatus/Lts/Viewer/LtsDataView/data_delegate.cpp

@@ -0,0 +1,31 @@
+/**
+ * @RDE_HEADER@
+ * Renders complex types from model data to text for views
+ * $Id$
+ * $HeadURL$
+ * $Author$
+ *
+ * Created by Mike Wille, NLE, 2020-09-09
+ */
+
+#include "data_delegate.h"
+
+#include <ngltools/ngltools_dtg.h>
+
+#include <QDateTime>
+
+
+namespace LtsDataView
+{
+DataDelegate::DataDelegate(QObject *parent)
+  : QStyledItemDelegate(parent)
+{}
+
+
+QString DataDelegate::displayText(const QVariant &value, const QLocale &locale) const
+{
+  if (value.type() == QVariant::Type::DateTime)
+    return DTG::toString(value.toDateTime());
+  return QStyledItemDelegate::displayText(value, locale);
+}
+} // namespace LtsDataView