Add appdata file for Thunar (Bug 11172)

Ryan Lerch wrote most of the appdata file, I only added the url
help and the autoconf stuff to use intltool for translated strings
in the appdata as we can do that as upstream. See:
http://www.freedesktop.org/software/appstream/docs/sect-Metadata-Application.html



Signed-off-by: Simon Steinbeiss <simon.steinbeiss@elfenbeinturm.at>