Revert "Don't run Quit() twice during startup"

This reverts commit b9fc3584.

the aboutToQuit signal is emitted just before the app closes and
gives chance to run some teardown routines, when closing the app
via a keyboard shortcut (cmd+q on macOS) we get this signal  and
the handler stops the firewall and performs other needed cleanup

removing this causes a bug where quiting the app would keep  the
firewall running which results into no internet access

gui/main.cpp

@@ -102,7 +102,6 @@ QList<QVariant> getAvailableLocales() {
 
 auto handler = [](int sig) -> void {
     printf("\nCatched signal(%d): quitting\n", sig);
-    Quit();
     QApplication::quit();
 };
 
@@ -136,6 +135,11 @@ int main(int argc, char **argv) {
     app.setQuitOnLastWindowClosed(false);
     app.setAttribute(Qt::AA_UseHighDpiPixmaps);
 
+    QObject::connect(&app, &QApplication::aboutToQuit, []() {
+            qDebug() << ">>> Quitting, bye!";
+            Quit();
+    });
+
 #ifdef OS_WIN
     signal(SIGINT, handler);
     signal(SIGTERM, handler);