Freeze break request for the control-center

[Matthias Clasen <matthias clasen gmail com>]

We have a new display panel in the control-center this cycle.
The early feedback we got on it indicates that many people miss the
'off' button. To avoid unnecessary blowback and controversy of the
'gnome always removes options, freedom!!!1111@" kind,
https://bugzilla.gnome.org/show_bug.cgi?id=707570 adds back an option
to turn a monitor off. I have tested the patch, it works fine.

The patch adds two new strings,

"Turn off"
"Don't use this display"