macOS: Remove popup mouse and app activation monitors on app shutdown

We're not guaranteed to get into any of the code paths that call
removePopupMonitor() before the app goes away. In a plugin-scenario,
this may cause crashes when our monitor then gets a callback and
we try to access QGuiApplicationPrivate::instance().

Pick-to: 6.6 6.5 6.2
Fixes: QTBUG-123959
Change-Id: I287b91ff261a8aab74adbbad8c63a042daf944d5
Reviewed-by: Doris Verria <doris.verria@qt.io>
(cherry picked from commit 8bb93bf8ee59b3e31e7353d6683dbb0aa890f7e7)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/cocoa/qcocoaintegration.mm

@@ -184,6 +184,9 @@ QCocoaIntegration::~QCocoaIntegration()
         [[NSApplication sharedApplication] setDelegate:nil];
     }
 
+    // Stop global mouse event and app activation monitoring
+    QCocoaWindow::removePopupMonitor();
+
 #ifndef QT_NO_CLIPBOARD
     // Delete the clipboard integration and destroy mime type converters.
     // Deleting the clipboard integration flushes promised pastes using