desktop should auto-save on idle time

[Sam Steingold]

Right now desktop autosave is disabled by default and, when enabled,
uses a regular timer.
Proposal: use idle timer instead and default `desktop-auto-save-timeout'
to `auto-save-timeout' (i.e., enable by default).
Objections?

PS. another pending change about to be committed is restoring
`buffer-display-time' for the benefit of `clean-buffer-list'.
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=15262

-- 
Sam Steingold (http://sds.podval.org/) on Ubuntu 13.04 (raring) X 11.0.11303000
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