emacs: info: Generalize inserting and formatting.

emacs: info: Generalize inserting and formatting.

* emacs/guix-info.el: Use a more flexible format for inserting any data.
  (guix-info-ignore-empty-vals): Rename to...
  (guix-info-ignore-empty-values): ... this.
  (guix-info-insert-methods): Merge this and...
  (guix-info-displayed-params): ... this into...
  (guix-info-format): ... this.  Change format specifications.
  (guix-info-title-aliases, guix-info-value-aliases): New variables.
  (guix-info-displayed-params): Adjust for the new format.
  (guix-info-insert-entry): Likewise.
  (guix-package-info-fill-heading): Replace with...
  (guix-info-fill): ... this.
  (guix-info-insert-param): Replace with...
  (guix-info-insert-entry-unit): ... this.
  (guix-info-insert-title-default): Replace with...
  (guix-info-insert-title-format): ... this.
  (guix-info-insert-val-default): Replace with...
  (guix-info-insert-value-format): ... this.
  (guix-info-insert-val-simple): Replace with...
  (guix-info-insert-value-indent): ... this.
  (guix-package-info-insert-source): Adjust accordingly.
  (guix-package-info-insert-heading): Insert only name and version.
  (guix-package-info-define-insert-inputs): Do not generate
  'guix-package-info-insert-ENTRY-TYPE-inputs' procedures.
  (guix-info-fill-column, guix-info-insert-entry-default)
  (guix-info-method-funcall, guix-info-insert-file-path)
  (guix-info-insert-url, guix-info-insert-package-function)
  (guix-info-insert-installed-function)
  (guix-info-insert-output-function)
  (guix-info-insert-generation-function)
  (guix-package-info-heading-params)
  (guix-package-info-insert-with-heading)
  (guix-package-info-insert-description)
  (guix-package-info-insert-location)
  (guix-package-info-insert-full-names)
  (guix-package-info-insert-source-url): Remove.
  (guix-info-fill-column, guix-info-param-title)
  (guix-info-title-function, guix-info-value-function)
  (guix-info-title-method->function)
  (guix-info-value-method->function)
  (guix-info-insert-value-simple): New procedures.
* emacs/guix-utils.el (guix-buttonize, guix-button-type?): New
  procedures.
  (guix-split-string): Split multi-line strings and ignore empty lines.
* doc/emacs.texi (Emacs Appearance): Adjust accordingly.