* button_gpl_down.svg
* button_gpl_normal.svg

Obtained September 2014 from http://www.gnu.org/graphics/gpl-v3-logo.svg
with permission:

  If you are releasing work under version 3 of the GNU GPL, LGPL, or
  AGPL, feel free to use these buttons on your site or in your
  application to advertise the license. These logos are immediately
  recognizable, and will assure your users that their freedom is being
  protected.

  (http://www.gnu.org/graphics/license-logos.html)

Changed to match application colour and indicate mouse hovering and
button clicks.
