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/tree_view_delegate.h

@@ -0,0 +1,33 @@
+#pragma once
+
+#include <LtsWidgets/lts_column_filter_proxy_model.h>
+
+#include <QStyledItemDelegate>
+#include <QStyle>
+
+
+namespace LtsDataView
+{
+class TreeViewDelegate final : public QStyledItemDelegate // needed to tell the QLineEdit to be as compact as possible while editing
+{
+  Q_OBJECT
+public:
+  using ComboBoxData = LtsWidgets::ComboboxFilterContent;
+  using QStyledItemDelegate::QStyledItemDelegate;
+
+  void paint(QPainter *painter,
+             const QStyleOptionViewItem &option, const QModelIndex &index) const override;
+  QWidget *createEditor(QWidget *parent, const QStyleOptionViewItem &option,
+                        const QModelIndex &index) const override;
+  void setEditorData(QWidget *editor, const QModelIndex &index) const override;
+  void setModelData(QWidget *editor, QAbstractItemModel *model,
+                    const QModelIndex &index) const override;
+
+  static QString noFilterString() { return tr("<No filter>"); }
+private:
+  static ComboBoxData comboBoxData(const QModelIndex &index);
+
+private slots:
+  void slotFinished();
+};
+} // namespace LtsDataView