Add centroid feedback to the WheelHandler autotest

wheelHandler.point.position is the centroid around which the item is
rotated or scaled.

Change-Id: I4993f8c6c9768dd18d1712b00354ea681bbad369
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

tests/auto/quick/pointerhandlers/qquickwheelhandler/data/rectWheel.qml

@@ -14,7 +14,23 @@ Rectangle {
         }
     }
 
+    Rectangle {
+        color: "red"
+        width: 6; height: 6; radius: 3
+        x: wheelHandler.point.position.x - radius
+        y: wheelHandler.point.position.y - radius
+    }
+
+    Text {
+        anchors.centerIn: parent
+        anchors.verticalCenterOffset: 20
+        color: "white"
+        font.pixelSize: 18
+        text: parent.x.toFixed(2) + ", " + parent.y.toFixed(2)
+    }
+
     WheelHandler {
+        id: wheelHandler
         activeTimeout: 0.5
     }
 }