adwaita-icon-theme: new package

This package provides the Adwaita icon theme from the GNOME desktop
environment.

[Thomas: fix the license information to indicate that CC-BY-SA-3.0 is
one of the licenses, and add the COPYING file as well.]

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>