This was proposed as an addition to the autostart spec here: http://lists.freedesktop.org/archives/xdg/2007-January/007436.html and well-received, but did not make it into the spec. However, gnome-session uses it, so we should a) add it to the spec b) make desktop-file-utils accept it
Fixed in git, with 31585ac26ff458159b01c6bcfcf1405e164d6448.
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