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

[Juri Linkov <juri@jurta.org>]

>> 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.
>
> Or, the desktop can be saved after 5 minutes (or whatever) of idle
> time, and if the user needs more frequency, they can bind
> desktop-save-in-desktop-dir and do it manually...

Or use the timer to display every 5 minutes a message
"Don't forget to type M-x desktop-save-in-desktop-dir RET" :-)

> When I'm writing or translating with LibreOffice, I do not trust
> autosaving. I type Ctrl-S after every paragraph. Seems saner.

Unlike LibreOffice, we can fix desktop.el in a way
that you'll trust Emacs autosaving :-)