QmlDesigner: Don't export members which are not used outside

Change-Id: I2eb756484a13ce81fd55e7b991d3a509045974e8
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/qmlprojectmanager/qmlproject.h

@@ -112,10 +112,9 @@ public:
     // plain format
     void parseProject(RefreshOptions options);
 
-    QPointer<QmlProjectItem> m_projectItem;
-    Utils::FilePath m_canonicalProjectDir;
-
 private:
+    std::unique_ptr<QmlProjectItem> m_projectItem;
+    Utils::FilePath m_canonicalProjectDir;
     bool m_blockFilesUpdate = false;
     friend class FilesUpdateBlocker;
 };