power: Default to suspend after 20 minutes of inactivity

According to the EU Commission Regulation No 801/2013, amending
Regulation (EC) No 1275/2008:
"
The default period of time after which the power management function,
or a similar function, switches the equipment automatically into a
condition providing networked standby shall not exceed 20 minutes.
"

(Full text at:
http://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:32013R0801&from=EN )

Though this addition only seems to be applicable to "networked
equipment", the original directive and its 2008 "ecodesign" regulation
mentions that computers are covered (as "Information technology equipment intended
primarily for use in the domestic environment"), and that:
"
When equipment is not providing the main function, or when other energy-using
product(s) are not dependent on its functions, equipment shall, unless
inappropriate for the intended use, offer a power management function, or a
similar function, that switches equipment after the shortest possible period
of time appropriate for the intended use of the equipment, automatically into
[standby mode].
"

(Full text at:
http://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:32008R1275&from=EN)

Furthermore, the EnergyStar standard that allows shipping into the USA
mentions that, for most computers types:
"
Sleep Mode shall be set to activate after no more than 30 minutes of
user inactivity.
"

(Full text at:
http://docketpublic.energy.ca.gov/PublicDocuments/16-AAER-02/TN213577_20160909T143121_ENERGY_STAR_Product_Specification_for_Computers_Eligibility_Cri.pdf)

https://bugzilla.gnome.org/show_bug.cgi?id=681869