Add pkexec policy (Bug #11122)

This adds a pkexec policy file to Thunar. This way if the user
of a desktop system wants to use thunar to modify files as root
and has the proper credentials they can.
pkexec comes from polkit which does about the same thing as
sudo/gksu except also supports authentication methods like
fingerprint or smart card readers.

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