xfce: Fix missing icons

Xfce didn't have a default value for its icon theme, so most icons
were missing until the user explicitly selected an icon theme in the
appearance settings.  Example:

  http://hydra.nixos.org/build/5717570/download/1/screen.png

This patch sets the Rodent theme as the default (which used to be the
case on NixOS).

http://lists.freebsd.org/pipermail/freebsd-ports/2012-June/076349.html
http://www.freebsd.org/cgi/query-pr.cgi?pr=168688