emacs: Generalize buffer redisplaying.

* emacs/guix-base.el (guix-buffer-after-redisplay-hook): New variable.
  (guix-redisplay-buffer): Use it.  Remove all arguments and the code
  for moving point to the next button.  Rename to...
  (guix-buffer-redisplay): ... this.
  (guix-buffer-redisplay-goto-button): New procedure.
* emacs/guix-info.el (guix-package-info-show-source): Use it.  Adjust
  accordingly.
  (guix-package-info-redisplay-after-download): Likewise.