emacs: Add support for displaying outputs.

emacs: Add support for displaying outputs.

Suggested by Taylan Ulrich Bayirli/Kammer and Ludovic Courtès.

* emacs/guix-base.el (guix-param-titles): Add output titles.
  (guix-messages): Add output messages.
  (guix-get-package-id-and-output-by-output-id): New procedure.
  (guix-define-buffer-type): Add ':buffer-name' key.
* emacs/guix-info.el: Add "output-info" buffer type.
  (guix-info-insert-methods): Add output methods.
  (guix-info-displayed-params): Add output params.
  (guix-output-info-insert-version, guix-output-info-insert-output): New
  procedures.
* emacs/guix-list.el: Add "output-list" buffer type.
  (guix-list-column-format): Add output formats.
  (guix-list-column-value-methods): Add output methods.
  (guix-package-list-type): New variable.
  (guix-generation-list-show-packages): Use it.
  (guix-package-list-marking-check): Use 'guix-output-list-mode'.
  (guix-list-mark-package-upgrades): New procedure.
  (guix-package-list-mark-upgrades): Use it.
  (guix-list-execute-package-actions): New procedure.
  (guix-package-list-execute): Use it.
  (guix-list-describe-maybe): New procedure.
  (guix-list-describe): Use it.
  (guix-output-list-mark-install, guix-output-list-mark-delete,
  guix-output-list-mark-upgrade, guix-output-list-mark-upgrades,
  guix-output-list-execute, guix-output-list-make-action,
  guix-output-list-describe): New procedures.
  (guix-output-list-describe-type): New variable.
* emacs/guix.el (guix-get-show-packages): Use 'guix-package-list-type'.
* doc/emacs.texi (emacs Commands): Mention 'guix-package-list-type'.
  (emacs List buffer): Adjust accordingly.
  (emacs Info buffer): Likewise.
  (emacs Buffer Names): New node.
  (emacs Keymaps): Add keymaps for output buffers.