services: Add LXQt desktop service.

* gnu/services/desktop.scm (lxqt-desktop-service-type): New variable.
(<lxqt-desktop-configuration>): New record type.
(lxqt-desktop-configuration?): New procedure.
* doc/guix.texi (Desktop Services): Document this.

Co-authored-by: Oleg Pykhalov <go.wigust@gmail.com>