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.cpp

@@ -0,0 +1,78 @@
+#include "icon_view_delegate.h"
+
+#include <QGraphicsDropShadowEffect>
+#include <QGraphicsScene>
+#include <QGraphicsPixmapItem>
+#include <QPainter>
+#include <QDebug>
+
+
+namespace LtsDataView
+{
+static constexpr int s_shadowRadius = 22;
+
+void IconViewDelegate::setDisplayWidgetCreator(const std::function<AbstractDisplayWidget*()> &customDisplayWidgetCreator)
+{
+  m_customDisplayWidgetCreator = customDisplayWidgetCreator;
+
+  QScopedPointer<AbstractDisplayWidget> dw(m_customDisplayWidgetCreator());
+  if (!dw)
+  {
+    qWarning() << "IconViewDelegate::setDisplayWidgetCreator: delegate is not a base of AbstractDisplayWidget!";
+    Q_ASSERT(false);
+    return;
+  }
+
+  m_displayWidgetSize = dw->size() + QSize(s_shadowRadius * 2, s_shadowRadius * 2);
+}
+
+
+QSize IconViewDelegate::sizeHint(const QStyleOptionViewItem &, const QModelIndex &) const
+{
+  return m_displayWidgetSize;
+}
+
+void IconViewDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option, const QModelIndex &index) const
+{
+  QScopedPointer<AbstractDisplayWidget> dw(m_customDisplayWidgetCreator());
+
+  if (!dw)
+  {
+    qWarning() << "IconViewDelegate::paint: delegate is not a base of AbstractDisplayWidget!";
+    Q_ASSERT(false);
+    return;
+  }
+
+  if (!dw->init(index))
+  {
+    qWarning() << "IconViewDelegate::paint: delegate could not create ui";
+    return;
+  }
+
+  const auto dpr = dw->devicePixelRatioF();
+  QPixmap p(dw->size() * dpr);
+  p.setDevicePixelRatio(dpr);
+  QPainter myPainter(&p);
+  dw->render(&myPainter);
+  myPainter.end();
+
+  // drop shadow
+  QGraphicsDropShadowEffect *shadowEffect = new QGraphicsDropShadowEffect();
+  shadowEffect->setBlurRadius(s_shadowRadius);
+  shadowEffect->setOffset(0, 0); // center (shadow on all sides)
+
+  if (option.state & QStyle::State_Selected)
+    shadowEffect->setColor(dw->palette().light().color());
+  else
+    shadowEffect->setColor(dw->palette().shadow().color());
+
+  // draw pixmap & shadow
+  QGraphicsPixmapItem *item = new QGraphicsPixmapItem(p);
+  item->setGraphicsEffect(shadowEffect);
+
+  // TODO: avoid creating a QGraphicsScene by using QGraphicsItem::paint()
+  QGraphicsScene scene;
+  scene.addItem(item);
+  scene.render(painter, option.rect, QRectF(-s_shadowRadius, -s_shadowRadius, option.rect.width(), option.rect.height()));
+}
+} // namespace LtsDataView