QtCore: replace uses of inefficient QList<QPair>s with QVectors [itemmodels]

These QPairs are larger than a void*, so holding them in QLists
is needlessly inefficient. Worse, the code could come to depend
on the fragile property of (inefficient) QLists that references
to elements therein never are invalidated.

Fix by holding them in QVector instead.

Change-Id: Ie8d8eff3448ada7aef5dfba7fc701a59821f5b54
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/itemmodels/qsortfilterproxymodel.cpp

@@ -54,7 +54,7 @@
 
 QT_BEGIN_NAMESPACE
 
-typedef QList<QPair<QModelIndex, QPersistentModelIndex> > QModelIndexPairList;
+typedef QVector<QPair<QModelIndex, QPersistentModelIndex> > QModelIndexPairList;
 
 static inline QSet<int> qVectorToSet(const QVector<int> &vector)
 {

src/corelib/itemmodels/qstringlistmodel.cpp

@@ -262,7 +262,7 @@ void QStringListModel::sort(int, Qt::SortOrder order)
 {
     emit layoutAboutToBeChanged(QList<QPersistentModelIndex>(), VerticalSortHint);
 
-    QList<QPair<QString, int> > list;
+    QVector<QPair<QString, int> > list;
     const int lstCount = lst.count();
     list.reserve(lstCount);
     for (int i = 0; i < lstCount; ++i)