installer: Desktop environment page now includes window managers.

* gnu/installer/services.scm (<system-service>)[snippet]: Change to be a
list of sexps and add default value.
[packages]: New field.
(%system-services): Adjust 'snippet' fields to be lists of sexps.
Add Openbox, awesome, i3, and ratpoison.
(system-services->configuration): Adjust 'snippet' handling.  Honor
'packages' field.