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.cpp

@@ -0,0 +1,118 @@
+/**
+  * @RDE_HEADER@
+  * Implementation of CheckStateProxyModel
+  * $Id$
+  * $HeadURL$
+  * $Author$
+  * Created by hjl/Christian Froebel, NLE, 2019-05-10
+  */
+
+#include "check_state_proxy_model.h"
+
+namespace LtsDataView
+{
+Qt::ItemFlags CheckStateProxyModel::flags(const QModelIndex &index) const
+{
+  if (!index.isValid())
+    return sourceModel()->flags(index);  // delegate to source model for further processing if necessary
+
+  // Sets only the checkbox nodes on user checkable,
+  // checkbox leaves must be handled in source model.
+  QModelIndex   sourceIndex = mapToSource(index);
+  Qt::ItemFlags   fl = sourceModel()->flags(sourceIndex);
+  if (index.column() == 0)
+    return fl | Qt::ItemIsUserCheckable;
+  return fl;
+}
+
+
+QVariant CheckStateProxyModel::data(const QModelIndex &index, int role) const
+{
+  if (!index.isValid())
+    return QVariant();
+
+  QModelIndex   sourceIndex = mapToSource(index);
+
+  // delegate everything except CheckStateRole to source model
+  if (role != Qt::CheckStateRole)
+    return sourceModel()->data(sourceIndex, role);
+
+  // delegate a leaf too with CheckStateRole to source model
+  if (isLeaf(index))
+    return sourceModel()->data(sourceIndex, role);
+
+  // delegate to source model for further processing of further columns if necessary
+  if (index.column() != 0)
+    return sourceModel()->data(sourceIndex, role);
+
+  // sets the node checkboxes and decoration depending on the entries in the tree structure
+  int checkedCount = 0;
+  int childCount = rowCount(index);
+  for (int childRow = 0; childRow < childCount; ++childRow)
+  {
+    QModelIndex      childIndex = this->index(childRow, 0, index);
+    Qt::CheckState   checkState = data(childIndex, Qt::CheckStateRole).value<Qt::CheckState>();
+    if (checkState == Qt::PartiallyChecked)
+      return Qt::PartiallyChecked;
+    if (checkState == Qt::Checked)
+      ++checkedCount;
+  }
+  if (checkedCount == childCount)
+    return Qt::Checked;
+  else if (checkedCount == 0)
+    return Qt::Unchecked;
+  else
+    return Qt::PartiallyChecked;
+}
+
+
+bool CheckStateProxyModel::setData(const QModelIndex &index, const QVariant &value, int role)
+{
+  if (!index.isValid())
+    return false;
+
+  QModelIndex  sourceIndex = mapToSource(index);
+
+  // delegate to source model for further processing of further columns if necessary
+  if (index.column() != 0)
+    return sourceModel()->setData(sourceIndex, value, role);
+
+  // delegate everything except CheckStateRole to source mode
+  if (role != Qt::CheckStateRole)
+    return sourceModel()->setData(sourceIndex, value, role);
+
+  if (isLeaf(index))
+  {
+    // delegate a leaf to source model
+    sourceModel()->setData(sourceIndex, value, role);
+
+    // update parents recursively here
+    QModelIndex idx = index.parent();
+    do
+    {
+      emit dataChanged(idx, idx, {Qt::CheckStateRole});
+      idx = idx.parent();
+    }
+    while (idx.isValid());
+    return true;
+  }
+
+  // update underlying children recursively (Additional nodes with leaves may also exist)
+  int childCount = rowCount(index);
+  for (int childRow = 0; childRow < childCount; ++childRow)
+  {
+    QModelIndex  childIndex = this->index(childRow, 0, index);
+    setData(childIndex, value, Qt::CheckStateRole);
+  }
+  return true;
+}
+
+
+bool CheckStateProxyModel::isLeaf(const QModelIndex &index) const
+{
+  QModelIndex sourceIndex = mapToSource(index);
+  return sourceModel()->flags(sourceIndex).testFlag(Qt::ItemIsUserCheckable);
+}
+
+
+}   // namespace InfrastructureManagement