emacs: Replace 'generation-diff' search with 'profile-diff'.

emacs: Replace 'generation-diff' search with 'profile-diff'.

* emacs/guix-main.scm (generation-package-specifications): Rename to...
  (profile-package-specifications): ... this.  Take a single 'profile'
  argument.
  (generation-difference): Rename to...
  (profile-difference): ... this.  Take profiles as arguments.
  (package/output-sexps): Adjust accordingly.
* emacs/guix-ui-generation.el (guix-generation-list-profiles-to-compare):
  New procedure.
  (guix-generation-list-show-added-packages)
  (guix-generation-list-show-removed-packages): Use it.
* emacs/guix-messages.el (guix-messages): Replace 'generation-diff' with
  'profile-diff'.
  (guix-message-outputs-by-diff): Adjust accordingly.