Do cleanup work in an earlier stage to ensure correct conf saving.

src/mainwindow.h

@@ -72,7 +72,6 @@ class MainWindow : public QMainWindow, private Ui::MainWindow{
 public:
   // Construct / Destruct
   MainWindow(QWidget *parent=0, const QStringList& torrentCmdLine = QStringList());
-  ~MainWindow();
   // Methods
   QWidget* getCurrentTabWidget() const;
   TransferListWidget* getTransferList() const { return transferList; }