Minimal config to get Network Manager Applet with usb modem support.

[Adam Mazurkiewicz]

I have been trying to run Network Manager Applet with my internet usb
modem for a long time, but I have been always getting a result
"Insufficient privileges" when trying to add the modem connection. Now
I have no idea how to achieve it or even debug. Please help me at
least in one of ways:
1. Show a config.scm file that is based on a default installing
desktop.scm file:

;; This is an operating system configuration template
;; for a "desktop" setup with GNOME and Xfce where the
;; root partition is encrypted with LUKS.

(use-modules (gnu) (gnu system nss))
(use-service-modules desktop)
(use-package-modules certs gnome)

(operating-system
  (host-name "antelope")
  (timezone "Europe/Paris")
  (locale "en_US.utf8")

  ;; Use the UEFI variant of GRUB with the EFI System
  ;; Partition mounted on /boot/efi.
  (bootloader (bootloader-configuration
                (bootloader grub-efi-bootloader)
                (target "/boot/efi")))

  ;; Specify a mapped device for the encrypted root partition.
  ;; The UUID is that returned by 'cryptsetup luksUUID'.
  (mapped-devices
   (list (mapped-device
          (source (uuid "12345678-1234-1234-1234-123456789abc"))
          (target "my-root")
          (type luks-device-mapping))))

  (file-systems (cons (file-system
                        (device (file-system-label "my-root"))
                        (mount-point "/")
                        (type "ext4")
                        (dependencies mapped-devices))
                      %base-file-systems))

2. Tell which is a common, universal way to install applications that
are popular for Desktop Environment like Xfce, Gnome etc.

Thanks