QTextDocumentLayout: don't hold QPointers in QList

QPointer is larger than a void*, so holding them in a QList 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.

Change-Id: I104d11cc530c9c1579e82bf4e28e3353c00b34b4
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/gui/text/qtextdocumentlayout.cpp

@@ -106,7 +106,7 @@ public:
     bool sizeDirty;
     bool layoutDirty;
 
-    QList<QPointer<QTextFrame> > floats;
+    QVector<QPointer<QTextFrame> > floats;
 };
 
 QTextFrameData::QTextFrameData()