* gnu/services/lightdm.scm: New service.
* tests/services/lightdm.scm: Test it.
* doc/guix.texi (X Window): Document it.
* gnu/local.mk (GNU_SYSTEM_MODULES): Register it.
Co-authored-by: L p R n d n <guix@lprndn.info>
Co-authored-by: Ricardo Wurmus <rekado@elephly.net>
