emacs: Generalize buffer naming.

* emacs/guix-base.el (guix-buffer-name): New procedure.
  (guix-buffer-define-interface): Make ':buffer-name' a required keyword.
  (guix-update-after-operation, guix-buffer-name-function)
  (guix-buffer-name-simple, guix-buffer-name-default, guix-buffer-name)
  (guix-buffer-p, guix-buffers, guix-update-buffer)
  (guix-update-buffers-maybe-after-operation): Adjust, move and rename to...
* emacs/guix-ui.el (guix-ui-update-after-operation)
  (guix-ui-buffer-name-function, guix-ui-buffer-name-simple)
  (guix-ui-buffer-name-default, guix-ui-buffer-name)
  (guix-ui-buffer?, guix-ui-buffers, guix-ui-update-buffer)
  (guix-ui-update-buffers-after-operation): ... this.
  (guix-ui-define-interface): Generate
  'guix-ENTRY-TYPE-BUFFER-TYPE-buffer-name' procedure and pass it as
  ':buffer-name' argument.
  (guix-ui): New custom group.
* emacs/guix-info.el: Specify ':buffer-name' for the defined interfaces.
* emacs/guix-list.el: Likewise.
* doc/emacs.texi (Emacs Appearance): Adjust accordingly.