Make it run with Qt6

This is a huge commit. Changes:
* Obviously, make it compile
* Make it run (only scarcely tested)
* get rid of most of clang's warnings

Let's see what surprises are in store...

1 files changed, 1 insertions, 1 deletions

diff --git a/mappingtreemodel.cpp b/mappingtreemodel.cpp
index 4f253c5..f88da64 100644
--- a/mappingtreemodel.cpp
+++ b/mappingtreemodel.cpp
@@ -205,7 +205,7 @@ bool MappingTreeModel::deleteChild(const QModelIndex &idx){
     return false;
 }
 
-MappingData MappingTreeModel::mappingDataFromItem(SmTreeItem *item) const{
+const MappingData MappingTreeModel::mappingDataFromItem(SmTreeItem *item) const{
     MappingData retval;
     retval.mappingId = item->data(MappingTreeModel::MappingId).toInt();
     retval.descId = item->data(MappingTreeModel::DescId).toInt();