emacs: list: Split 'guix-list-format' variable.

* emacs/guix-list.el: Generate 'guix-ENTRY-TYPE-list-format' variables
  for 'package', 'output' and 'generation' entry types.
  (guix-list-format): Remove.
  (guix-list-define-entry-type): Add ':format' keyword.
* doc/emacs.texi (Emacs Appearance): Adjust accordingly.