Improve performance of QItemSelection, -SelectionModel and -SelectionRange with Qt::ItemIsSelectable flag

QItemSelection, QItemSelectionModel and QItemSelectionRange could improve performance by using Qt::ItemIsSelectable flag to a greater extent.
Currently indexes passed in to the model may not be selectable and many function calls can therefore be saved by checking whether these indexes are selectable or not when inside functions like QItemSelectionModel::rowIntersectsSelection() and QItemSelectionModel::contains().