installer: Generalize desktop environments to system services.
* gnu/installer/services.scm (<desktop-environment>): Rename to...
(<system-service>): ... this. Add a 'type' field.
(%desktop-environments): Rename to...
(%system-services): ... this.
(desktop-system-service?): New procedure.
(desktop-environments->configuration): Rename to...
(system-services->configuration): ... this. Determine the base list of
services based on whether SERVICES contains at least one "desktop"
service.
* gnu/installer/newt/services.scm (run-desktop-environments-cbt-page):
Adjust accordingly.
* gnu/installer.scm (installer-steps): Likewise.