Doc: Let SQL Drill Down example show correct use of qgraphicsitem_cast

For qgraphicsitem_cast to work correctly with custom items, one must
reimplement the type() function for each custom QGraphicsItem subclass.
Do that in the example code.

Fixes: QTBUG-7357
Change-Id: Ibf221817908355f0347cddfc46747b6955958f5f
Reviewed-by: Andy Shaw <andy.shaw@qt.io>

examples/sql/doc/src/drilldown.qdoc

@@ -437,6 +437,9 @@
 
     \snippet drilldown/imageitem.h 0
 
+    We declare a \c Type enum value for our custom item and reimplement
+    \l{QGreaphicsItem::}{type()}. This is done so we can safely use
+    qgraphicsitem_cast().
     In addition, we implement a public \c id() function to be able to
     identify the associated location and a public \c adjust() function
     that can be called to ensure that the image item is given the

examples/sql/drilldown/imageitem.h

@@ -60,8 +60,11 @@ class ImageItem : public QObject, public QGraphicsPixmapItem
     Q_OBJECT
 
 public:
+    enum { Type = UserType + 1 };
+
     ImageItem(int id, const QPixmap &pixmap, QGraphicsItem *parent = nullptr);
 
+    int type() const override { return Type; }
     void adjust();
     int id() const;