ProjectNodes: Sprinkle override over the Node classes

Change-Id: I87db3a1b212aa0a3f140b8d299679f6c1f691004
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/pythoneditor/pythoneditorplugin.cpp

@@ -778,7 +778,7 @@ public:
         , m_displayName(nodeDisplayName)
     {}
 
-    QString displayName() const { return m_displayName; }
+    QString displayName() const override { return m_displayName; }
 private:
     QString m_displayName;
 };