emacs: Add customization groups for faces.

* emacs/guix.el (guix-faces): New custom group.
* emacs/guix-base.el (guix-operation-option-key): Use it.
  (guix-define-buffer-type): Generate faces group.
* emacs/guix-info.el: Adjust faces to use new groups.
  (guix-info-faces): New custom group.
* emacs/guix-list.el: Likewise
  (guix-list-faces): New custom group.