gparted: add polkit support

* This was missing for me and is a good test case for xfce-polkit asking for
  root's password.
* Decision to use polkit (installed?) is made at runtime. Unfortunately this
  cool solution was not my idea but stolen from [1]

[1] http://pkgs.fedoraproject.org/cgit/gparted.git



Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>