BUG: Replace improperly licensed icons

Replace icons which were taking from the Oxygen theme and being used in
violation of their license (LGPLv3, which would require us to at minimum
be providing the scalable versions) with icons from the Tango theme,
instead, which are Public Domain.

Note that the statement in r21618 that the icons were licensed under
CC-BY-SA 3.0 appears to be incorrect. While that claim is indeed made by
Open Icon Library, I was unable to find any evidence to substantiate it,
and all other sources (including the canonical kde.org distribution
tarball) give LGPLv3 as the only applicable license.

See also http://techbase.kde.org/Projects/Oxygen/Licensing,
http://commons.wikimedia.org/wiki/Tango_icons and
http://en.wikipedia.org/wiki/Tango_Desktop_Project.

PNG's copied from tango-icon-theme-0.8.90-6.fc17.noarch.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23146 3bd1e089-480b-0410-8dfb-8563597acbee