systemtray: Fix race in DBusServiceObserver
When a service appears right after the ListNames() call finished, but before the QDBusPendingCallWatcher is set up, the ::serviceRegistered signal will be queued before the ListNames() response. As a result, the service will be ignored by DBusServiceObserver.
Switching to QDBusInterface::callWithCallback() ensures that the result slot is queued as soon as the response arrives, thus avoiding the race.
Credits go to Sam Edwards for identifying the problem in https://bugs.kde.org/show_bug.cgi?id=422111#c69