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.
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