Fix addGroup to return the integer groupIndex other methods expect

This commit fixes #6591, although doesn't make the existing
interface any better than it is, and it is really bad :)
No plugin should need any adaption with the changes.

src/app/legend/qgslegend.cpp

@@ -230,7 +230,18 @@ int QgsLegend::addGroup( QString name, bool expand, QTreeWidgetItem* parent )
 
   emit itemAdded( groupIndex );
 
-  return groupIndex.row();
+  // TODO: use QModelIndex::iternalId for an identifier instead of this mess ?
+  int itemCount = 0;
+  for ( QTreeWidgetItem* theItem = firstItem(); theItem; theItem = nextItem( theItem ) )
+  {
+    QgsLegendItem* legendItem = dynamic_cast<QgsLegendItem *>( theItem );
+    if ( legendItem->type() == QgsLegendItem::LEGEND_GROUP )
+    {
+      if ( legendItem == group ) return itemCount;
+      else itemCount++;
+    }
+  }
+  return itemCount; // bogus return
 }
 
 int QgsLegend::addGroup( QString name, bool expand, int groupIndex )