Qt6 regression in QGraphicsScene due to QList refactoring: removeItem() is no more a constant time operation in a scene with no index

Hi,

In Qt5, QGraphicsScene::removeItem() was a constant time operation when the index method was set to 'NoIndex', as stated in the documentation. This efficiency was due to the method QGraphicsSceneLinearIndex::removeItem() which simply removed the indexed items from a QList (constant time operation in Qt5).

Now in Qt6, removing an item from a QList is not a constant time operation any more (due to the QList refactoring). As a consequence, QGraphicsSceneLinearIndex should store indexed items in another container than a QList (like std::list for instance) in order to recover an efficient removeItem() method as it was in Qt5.