services: '%desktop-services' uses NetworkManager instead of Wicd.

Discussed at
<https://lists.gnu.org/archive/html/guix-devel/2017-08/msg00277.html>.

* gnu/services/desktop.scm (%desktop-services): Remove call to
'wicd-service'.  Add instances of NETWORK-MANAGER-SERVICE-TYPE and
WPA-SUPPLICANT-SERVICE-TYPE.
* doc/guix.texi (Networking Services): Document
'network-manager-service-type' as being part of '%desktop-services'.
(Desktop Services): Replace Wicd with NM.

doc/guix.texi

@@ -10010,6 +10010,9 @@ This is the service type for the
 @uref{https://wiki.gnome.org/Projects/NetworkManager, NetworkManager}
 service. The value for this service type is a
 @code{network-manager-configuration} record.
+
+This service is part of @code{%desktop-services} (@pxref{Desktop
+Services}).
 @end defvr
 
 @deftp {Data Type} network-manager-configuration
@@ -11573,7 +11576,7 @@ adds or adjusts services for a typical ``desktop'' setup.
 
 In particular, it adds a graphical login manager (@pxref{X Window,
 @code{slim-service}}), screen lockers, a network management tool
-(@pxref{Networking Services, @code{wicd-service}}), energy and color
+(@pxref{Networking Services, @code{network-manager-service-type}}), energy and color
 management services, the @code{elogind} login and seat manager, the
 Polkit privilege service, the GeoClue location service, the
 AccountsService daemon that allows authorized users change system

gnu/services/desktop.scm

@@ -809,8 +809,9 @@ with the administrator's password."
          (simple-service 'mtp udev-service-type (list libmtp))
 
          ;; The D-Bus clique.
+         (service network-manager-service-type)
+         (service wpa-supplicant-service-type)    ;needed by NetworkManager
          (avahi-service)
-         (wicd-service)
          (udisks-service)
          (upower-service)
          (accountsservice-service)