lp1912129: Always destroy MixxxMainWindow before CoreServices

Fixes [lp1912129].

This gets rid of an issue where Mixxx failed with an assertion during
shutdown that complained about leaked controls (that are owned by the
menu bar):

    warning [Main] The following 6 controls were leaked:
    warning [Main] "[VinylControl]" "show_vinylcontrol" QObject(0x0)
    warning [Main] "[Master]" "skin_settings" QObject(0x0)
    warning [Main] "[PreviewDeck]" "show_previewdeck" QObject(0x0)
    warning [Main] "[Master]" "maximize_library" QObject(0x0)
    warning [Main] "[Library]" "show_coverart" QObject(0x0)
    warning [Main] "[Microphone]" "show_microphone" QObject(0x0)
    DEBUG ASSERT: "!"Controls were leaked!"" in function void mixxx::CoreServices::shutdown() at /home/jan/Projects/mixxx/src/coreservices.cpp:582
    Aborted (core dumped)

The issue occurs when shutting down Mixxx by pressing <Super>+q in Pop!
Shell which closes the window immediately. Apparently this is
incomaptible with our `sendPostedEvents()` trick to destroy the menubar
and leads to `CoreServices` being shut down before the Menubar is
actually destroyed, leaving some COs intact and triggering the
assertion.

This fixes the issue by ensuring that `MixxxMainWindow` is always
completely destroyed before `CoreServices::shutdown()` is called.

[lp1912129]: https://bugs.launchpad.net/mixxx/+bug/1912129

src/main.cpp

@@ -27,28 +27,36 @@ int runMixxx(MixxxApplication* pApp, const CmdlineArgs& args) {
 
     CmdlineArgs::Instance().parseForUserFeedback();
 
-    MixxxMainWindow mainWindow(pCoreServices);
-    pApp->processEvents();
-    pApp->installEventFilter(&mainWindow);
-
-    QObject::connect(pCoreServices.get(),
-            &mixxx::CoreServices::initializationProgressUpdate,
-            &mainWindow,
-            &MixxxMainWindow::initializationProgressUpdate);
-    pCoreServices->initialize(pApp);
-    mainWindow.initialize();
-
-    // If startup produced a fatal error, then don't even start the
-    // Qt event loop.
-    if (ErrorDialogHandler::instance()->checkError()) {
-        return kFatalErrorOnStartupExitCode;
-    } else {
-        qDebug() << "Displaying main window";
-        mainWindow.show();
-
-        qDebug() << "Running Mixxx";
-        return pApp->exec();
+    int exitCode;
+
+    // This scope ensures that `MixxxMainWindow` is destroyed *before*
+    // CoreServices is shut down. Otherwise a debug assertion complaining about
+    // leaked COs may be triggered.
+    {
+        MixxxMainWindow mainWindow(pCoreServices);
+        pApp->processEvents();
+        pApp->installEventFilter(&mainWindow);
+
+        QObject::connect(pCoreServices.get(),
+                &mixxx::CoreServices::initializationProgressUpdate,
+                &mainWindow,
+                &MixxxMainWindow::initializationProgressUpdate);
+        pCoreServices->initialize(pApp);
+        mainWindow.initialize();
+
+        // If startup produced a fatal error, then don't even start the
+        // Qt event loop.
+        if (ErrorDialogHandler::instance()->checkError()) {
+            exitCode = kFatalErrorOnStartupExitCode;
+        } else {
+            qDebug() << "Displaying main window";
+            mainWindow.show();
+
+            qDebug() << "Running Mixxx";
+            exitCode = pApp->exec();
+        }
     }
+    return exitCode;
 }
 
 } // anonymous namespace