* lisp/desktop.el (desktop-auto-save-timeout): New option.
(desktop-file-checksum): New variable.
(desktop-save): Add optional arg `auto-save' and don't auto-save
if nothing changed.
(desktop-auto-save-timer): New variable.
(desktop-auto-save, desktop-auto-save-set-timer): New functions.
(after-init-hook): Call `desktop-auto-save-set-timer'.
Suggested by Reuben Thomas <rrt@sc3d.org> in
<http://lists.gnu.org/archive/html/emacs-devel/2013-04/msg00327.html>.