QMdiArea: replace a QList<int> with a QVector

A QVector<int> uses 50% less space on 64-bit platforms.

Change-Id: I6a2174b04b362188597bc42cad84b7ce2018ea78
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/widgets/widgets/qmdiarea_p.h

@@ -144,7 +144,7 @@ public:
     QList<QMdi::Rearranger *> pendingRearrangements;
     QVector< QPointer<QMdiSubWindow> > pendingPlacements;
     QVector< QPointer<QMdiSubWindow> > childWindows;
-    QList<int> indicesToActivatedChildren;
+    QVector<int> indicesToActivatedChildren;
     QPointer<QMdiSubWindow> active;
     QPointer<QMdiSubWindow> aboutToBecomeActive;
     QBrush background;