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.