Function QItemSelectionModel::isSelected should be virtual

After changing QItemSelectionModel::selected(...) for

After changing the QItemSelectionModel::select() functions may need to change the functions check whether an item is selected. Otherwise QItemSelectionModel::isSeletsted(...) limits the possible logic that could be implemented in QItemSelectionModel::select(...). When working with large trees (hundreds of thousands of items) should avoid the use selection of ranges and use selection of branches, otherwise have very poor performance. When working with selections based on the branches, the default behavior QItemSelectionModel::isSelected(...) not suitable.