Stop the autosaving session timer on closing Creator

Change-Id: I3b9c791d95d7815a711f72cec08f405fb1cdac7c
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>

src/plugins/coreplugin/mainwindow.h

@@ -232,6 +232,7 @@ private:
     QColor m_overrideColor;
 
     QStringList m_filesToOpenDelayed;
+    QTimer *m_autoSaveSessionTimer;
 };
 
 } // namespace Internal