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/abstract_display_widget.h

@@ -0,0 +1,30 @@
+/**
+ * @RDE_HEADER@
+ * Abstract Display Widget is an Abstract class to create Custom Display Widgets for the Custom View
+ * $Id$
+ * $HeadURL$
+ * $Author$
+ *
+ * Created by Mike Wille, SLSE, 2016-06-15
+ */
+#pragma once
+
+#include "lts_data_view_export.h"
+
+#include <QWidget>
+class QModelIndex;
+
+namespace LtsDataView
+{
+class LTSDATAVIEW_EXPORT AbstractDisplayWidget : public QWidget
+{
+  Q_OBJECT
+
+public:
+  explicit AbstractDisplayWidget(QWidget *parent = nullptr);
+  virtual bool init(const QModelIndex &idx) = 0;
+
+protected:
+  QVariant rowData(const QModelIndex &idx, int column, int role = Qt::DisplayRole) const;
+};
+} // namespace LtsDataView