Fix dead link to valid Oxygen icon source

The oxygen-icons.org website is gone.  Did some research and discovered the KDE Oxygen icon set with a CC-BY-SA 3.0 or LGPL License once distributed by oxygen-icons.org, is now being distributed by https://sourceforge.net/projects/openiconlibrary with the blessing of prominent GNU/Linux/KDE developer Jonathan Riddell: http://comments.gmane.org/gmane.comp.kde.licensing/467.