services: sddm, slim, gdm: Take an <xorg-configuration> record.

* gnu/services/sddm.scm (<sddm-configuration>)[xorg-server-path]
[xserver-arguments]: Remove.
[xorg-configuration]: New field.
(sddm-configuration-file): Adjust accordingly.
* gnu/services/xorg.scm (<slim-configuration>)[startx]: Remove.
[xorg-configuration]: New field.
(slim-shepherd-service, slim-service): Adjust accordingly.
(<gdm-configuration>)[x-server]: Remove.
[xorg-configuration]: New field.
(gdm-shepherd-service, gdm-service): Adjust accordingly.
* doc/guix.texi (X Window): Update accordingly.