Fix Qt 6 definition of macro ‘Q_DECLARE_MOVABLE_CONTAINER’

adding #include <QHash>

+ code fail to compile if it uses deprecated APIs.
=> see .pro file

sources/ElementsCollection/elementscollectionmodel.h

@@ -19,6 +19,7 @@
 #define ELEMENTSCOLLECTIONMODEL2_H
 
 #include <QStandardItemModel>
+#include <QHash>
 #include "elementslocation.h"
 
 class XmlProjectElementCollectionItem;