Quit properly on system shutdown

If you shutting down your PC, this won't save your session information. But it is working well when you are exiting from systray. I left my PC often with shutdown /s /t {one-two hours here} so my movies or whatever would finish downloading. But in this case, I cannot rely on Soulseek to properly save downloaded files info (about uploaded we do not really care - downloaders would re-request it next time).