This should allow more operations such as changing the system date and
time via the Control Center (reported at
<https://lists.gnu.org/archive/html/help-guix/2016-09/msg00059.html>),
performing "file operations" via gvfs, and interacting with processes
using the System Monitor.
* gnu/services/desktop.scm (gnome-polkit-settings): New procedure.
(gnome-desktop-service-type): Use it in POLKIT-SERVICE-TYPE extension.
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