dbustray: Delete m_notifier in QDBusTrayIcon::cleanup()

This fixes a bug where notificationClosed() and actionInvoked() slots
were called more than once, from previous alive notifier instances.

Change-Id: I4cb4dfc27ee129bc5282fbd8e0961959d0765112
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/platformsupport/dbustray/qdbustrayicon.cpp

@@ -118,6 +118,8 @@ void QDBusTrayIcon::cleanup()
         dBusConnection()->unregisterTrayIcon(this);
     delete m_dbusConnection;
     m_dbusConnection = Q_NULLPTR;
+    delete m_notifier;
+    m_notifier = Q_NULLPTR;
     m_registered = false;
 }