Hi all!

This patch fixes an issue in the XFCE desktop service configuration which causes the polkit action for xfce4-power-manager to not be installed in /etc/polkit-1/actions.  The lack of this file causes the user to be prompted for administrator credentials when they attempt to use their laptop's screen brightness keys.

The fix is to use a similar pattern to the GNOME service and add an 'xfce-polkit-settings' procedure which generates the list of XFCE packages that contain polkit actions to install.  Now that this list contains 'xfce4-power-manager`, the polkit file 'org.xfce.power.policy' is installed correctly and the screen brightness keys work as intended.

Thanks!

David