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/check_state_proxy_model.h

@@ -0,0 +1,60 @@
+/**
+  * @RDE_HEADER@
+  * The model is used to handle CheckState Boxes in an LtsDataView (QTreeModel).
+  * Only the nodes of a TreeView are handled in this model. Leaves are forwarded to the transferred model and must be edited there.
+  * $Id$
+  * $HeadURL$
+  * $Author$
+  * Created by hjl/Christian Froebel, NLE, 2019-05-10
+  */
+
+#pragma once
+
+#include <QIdentityProxyModel>
+
+#include "lts_data_view_export.h"
+
+namespace LtsDataView
+{
+
+class LTSDATAVIEW_EXPORT CheckStateProxyModel : public QIdentityProxyModel
+{
+public:
+  using QIdentityProxyModel::QIdentityProxyModel;
+
+  /**
+    * Handles the 'ItemIsUserCheckable' flags
+    * @param[in] sourceModel   index
+    *
+    */
+  Qt::ItemFlags flags(const QModelIndex &index) const override;
+
+  /**
+    * Handles the representation of data
+    * @param[in] sourceModel   index
+    * @param[in] role          role to which the data relates
+    *
+    */
+  QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override;
+
+  /**
+    * Handles the storage of data
+    * @param[in] index   index
+    * @param[in] value   value to be set
+    * @param[in] role    role to which the data relates
+    *
+    */
+  bool setData(const QModelIndex &index, const QVariant &value, int role = Qt::EditRole) override;
+
+private:
+
+  /**
+    *  Returns true if the item with the given index is an leaf
+    * @param[in] index    item index whose information is requested
+    *                     will be internally mapped to source model data
+    */
+  bool isLeaf(const QModelIndex &index) const;
+
+}; // class CheckStateProxyModel
+} // namespace LtsDataView
+