QColumnView: re-enable scrolling of the preview widget

For some reason, each column in a QColumnView is a QAbstractItemView,
*including* the preview widget's column. Unfortunately,
the preview widget's column class was not overriding scrollContentsBy,
so scrolling it had no effect.

A more comprehensive solution would be a major refactoring of the
code to make that column a plain Q(Abstract)ScrollArea, as it doesn't
need QAIV's APIs at all, but I don't want to change code and risk
breaking behavior.

Change-Id: Ice500a8eaef13c295df4cc274b9f80d9a24c65f4
Task-number: QTBUG-11392
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Alexander Volkov <a.volkov@rusbitech.ru>
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>

src/widgets/itemviews/qcolumnview_p.h

@@ -89,6 +89,16 @@ public:
         QAbstractScrollArea::resizeEvent(event);
     }
 
+    void scrollContentsBy(int dx, int dy) Q_DECL_OVERRIDE
+    {
+        if (!previewWidget)
+            return;
+        scrollDirtyRegion(dx, dy);
+        viewport()->scroll(dx, dy);
+
+        QAbstractItemView::scrollContentsBy(dx, dy);
+    }
+
     QRect visualRect(const QModelIndex &) const Q_DECL_OVERRIDE
     {
         return QRect();