QmlDesigner: Add new function to NodeInstanceView mockup

Model now uses requestModelNodePreviewImage so it's needed in mockup.

Change-Id: I9f9e43b288401de52ac010c333591c9184e10366
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

tests/unit/mockup/qmldesigner/designercore/include/nodeinstanceview.h

@@ -85,6 +85,8 @@ public:
     void importsChanged(const QList<Import> &addedImports, const QList<Import> &removedImports) override
     {}
 
+    void requestModelNodePreviewImage(const ModelNode &node) {}
+
     void sendToken(const QString &token, int number, const QVector<ModelNode> &nodeVector) {}
 };