QmlDesigner: Show the scene id in 3D edit view

It's sometimes hard to tell which scene is shown in 3D edit view, so
added the scene id to the upper left corner of the view.

Task-number: QDS-2239
Change-Id: I5999e41c3f92028cd9a0f79ae0c8b72263d9ce42
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

share/qtcreator/qml/qmlpuppet/mockfiles/EditView3D.qml

@@ -645,5 +645,15 @@ Item {
             editCameraCtrl: cameraControl
             selectedNode : viewRoot.selectedNodes.length ? selectionBoxes[0].model : null
         }
+
+        Text {
+            id: sceneLabel
+            text: viewRoot.sceneId
+            anchors.top: parent.top
+            anchors.left: parent.left
+            anchors.margins: 4
+            font.pixelSize: 14
+            color: "white"
+        }
     }
 }