make slots normal functions

src/gui/mainwindow.h

@@ -17,12 +17,11 @@ class MainWindow : public QMainWindow
     MainWindow();
     Marketplace* getMarketplace() { return marketplace_.get(); }
 
-  private slots:
+  private:
     void on_actionEditSeller_triggered();
     void on_sellerNoEdit_checkSellerNo();
     void on_paidButton_triggered();
 
-  private:
     Ui::MainWindow ui_;
     std::unique_ptr<Marketplace> marketplace_;
 };