[startkde] Block plasma-session exiting until startup sound completes
The initial comment about disabling quitLock is deprecated since we added a QCoreApplication around the startup.
We can then use Qt's mechanism to delay application exit till the notification thread finishes.
This presumably has been a bug for a while, but noticable in 5.19 after even more startup performance improvements.
Do not cherry-pick this to 5.18 as the comment in main.cpp was relevant then.