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box, the list will get narrowed to just the Keyboard properties.
Launch it.

Go to Shortcuts->System, then scroll down to "Log Out"
You will see it assigned to "Ctrl-Alt-Delete". Click on it, the value
will change from "Ctrl-Alt-Del" to "New accelerator"
Now press the desired hotkey to assign for the logout screen
(I choose [Ctrl] + [-] on the keypad, which feels natural as the
keypad minus also has "Z z z z" printed as secondary function. :)