emacs: Factorize macros for defining interfaces.

Make a root 'guix-buffer-define-interface' macro.  It should generate a
common code for any type of interface.  Inherit 'guix-info-define-interface'
and 'guix-list-define-interface' from it.  They should provide a general
'info'/'list' interface for any data.  Finally, make
'guix-ui-define-interface' for the common code for interfaces to Guix
packages and generations, and inherit 'guix-ui-info-define-interface' and
'guix-ui-list-define-interface' from it.

* emacs/guix-base.el (guix-define-buffer-type): Rename to...
  (guix-buffer-define-interface): ... this.  Rename internal
  variables ('buf-' -> 'buffer-').  Move ':required' keyword to
  'guix-ui-define-interface'.
* emacs/guix-info.el (guix-info-define-interface): New macro.
  (guix-info-font-lock-keywords): New variable.
* emacs/guix-list.el (guix-list-define-entry-type): Rename to...
  (guix-list-define-interface): ... this.
  (guix-list-font-lock-keywords): New variable.
  (guix-list-describe-ids): Move and rename to...
* emacs/guix-ui.el: New file.
  (guix-ui-list-describe): ... this.
  (guix-ui-define-interface, guix-ui-info-define-interface)
  (guix-ui-list-define-interface): New macros.
  (guix-ui-font-lock-keywords): New variable.
* emacs.am (ELFILES): Add "emacs/guix-ui.el"