make login and logout sound work with gnome-session-canberra

Yousuf Philips requested to merge philipzae/xdg-sound-theme:patch-1 into master

gnome-session-canberra is looking to play desktop-login.oga and desktop-logout.oga so smylinking it to service-login.oga and service-logout.oga. Also some sound themes use system-ready.oga and system-shutdown.oga, so adding symlinks for those as well.

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