Show only notifications for macOS system tray, not constant menubar icon

Otherwise, a menubar QGIS icon is shown, but contains no child actions.

This method of showing notifications should probably be moved to an
abstract call. However, on macOS, the showMessage() icon and timeout
are ignored.

Better to use a direct Obj-C++ method for macOS notifications:
https://wiki.qt.io/How_to_use_OS_X_Notification_Center

src/app/qgisapp.cpp

@@ -1131,6 +1131,11 @@ QgisApp::QgisApp( QSplashScreen *splash, bool restorePlugins, bool skipVersionCh
   mTray = new QSystemTrayIcon();
   mTray->setIcon( QIcon( QgsApplication::appIconPath() ) );
   mTray->show();
+#ifdef Q_OS_MAC
+  // We only want the system notifications, no need for constant menubar icon
+  mTray->hide();
+#endif
+
 
   connect( QgsApplication::taskManager(), &QgsTaskManager::statusChanged, this, &QgisApp::onTaskCompleteShowNotify );
 
@@ -11433,7 +11438,15 @@ void QgisApp::onTaskCompleteShowNotify( long taskId, int status )
     if ( task )
     {
       QString description = task->description();
+#ifdef Q_OS_MAC
+      // Menubar icon is hidden on macOS, by default. Show to enable notification bubbles
+      mTray->show();
+#endif
       mTray->showMessage( "Task Complete", description );
+#ifdef Q_OS_MAC
+      // Re-hide menubar icon
+      mTray->hide();
+#endif
     }
   }
 }