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/icon_view_delegate.h

@@ -0,0 +1,26 @@
+#pragma once
+
+#include "abstract_display_widget.h"
+
+#include <QStyledItemDelegate>
+
+namespace LtsDataView
+{
+class IconViewDelegate final : public QStyledItemDelegate
+{
+  Q_OBJECT
+public:
+  using QStyledItemDelegate::QStyledItemDelegate;
+  void setDisplayWidgetCreator(const std::function<AbstractDisplayWidget*()> &customDisplayWidgetCreator);
+
+  QSize sizeHint(const QStyleOptionViewItem &option,
+                 const QModelIndex &index) const;
+  void paint(QPainter *painter,
+             const QStyleOptionViewItem &option,
+             const QModelIndex &index) const;
+
+private:
+  std::function<AbstractDisplayWidget*()> m_customDisplayWidgetCreator;
+  QSize m_displayWidgetSize;
+};
+} // namespace LtsDataView