qt: Reenable and make functional ACPI shutdown button

src/qt/qt_mainwindow.hpp

@@ -134,6 +134,9 @@ protected:
     void closeEvent(QCloseEvent *event) override;
     void changeEvent(QEvent *event) override;
 
+private slots:
+    void on_actionACPI_Shutdown_triggered();
+
 private slots:
     void on_actionShow_non_primary_monitors_triggered();