[HIG] Specifying the desktop as user space only

[Dave Bordoley <bordoley msu edu>]

This relates to chapter 2 -- desktop integration:

I would like to suggest that we explicitly ban apps from adding icons
for launchers etc. to the desktop directory and thus proclaim this area
of the screen user space only. This hasn't been a problem to date as far
as I know, but in the future this may become an issue (the situation on
windows showcases how terrible this could become).

dave