startkde/plasma-session: fix playing startup sound when not using the systemd startup
Attaching the Phonon objects to the Startup object and then immediately deleting the Startup object leads to the Phonon objects being deleted too and no sound will play.
I have no idea how that ever worked, maybe Phonon used to work differently in the past or maybe it depends on the backend.
Now the Phonon objects are attached to the SessionTrack object, which will live for the whole session.