Generate 'guix-ENTRY-TYPE-BUFFER-TYPE-titles' variables for each
ENTRY-TYPE and BUFFER-TYPE instead.

* emacs/guix-base.el (guix-param-titles): Remove.
  (guix-buffer-data): New variable.
  (guix-buffer-value, guix-buffer-param-title): New procedures.
  (guix-buffer-define-interface): Add ':titles' keyword.
* emacs/guix-info.el (guix-info-param-title): Adjust accordingly.
* emacs/guix-list.el (guix-list-param-title): Likewise.
  (guix-list-column-titles): Remove.
* doc/emacs.texi (Emacs Appearance): Adjust accordingly.