bug#15331: desktop should auto-save on idle time

[Juri Linkov]

> I think the proposed solution is overly complicated.
> I think the better solution is to use idle timer instead and default
> `desktop-auto-save-timeout' to `auto-save-timeout' (i.e., enable by default).

You can't use idle timers even with such reasonable timeouts
as 5 minutes between saves, i.e. when the user needs to save
the desktop every 5 minutes, with idle timers the user have to sit
inactively 5 minutes waiting for Godot^H^H^H^H^H desktop saving.
Thus desktop saving requires two different timers out of necessity.

You can see the whole relevant emacs-devel thread
where this problem was discussed here:
http://thread.gmane.org/gmane.emacs.bugs/17096/focus=158816