services: Add auto-enable? parameter to the bluetooth-service.

* gnu/services/desktop.scm (bluetooth-configuration): New record.
(bluetooth-shepherd-service): Use it.
(bluetooth-directory): New method.
(bluetooth-service-type): Use it to extend the etc-service-type service.
(bluetooth-service): Add `auto-enable?' parameter.
* doc/guix.texi (Desktop Services): Document it.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>