bug#49330: Customization buffer should also offer printing code snippets

[積丹尼 Dan Jacobson]

The *Customize Face* buffer looks like:

  For help using this buffer, see Easy Customization in the Emacs manual.

  Operate on all settings in this buffer:
  Revert...   Apply   Apply and Save
  1^^^^^^^^^^^2^^^^^^^3^^^^^^^^^^^^^

OK, but add a fourth menu item:
"Print code snippet" or
"Just print code snippet" or just
"Print"!

Else currently one must
1. cp .emacs-custom.el /tmp
2. click "Apply and Save"
3. exit emacs #as we are about to replace a dot file it uses.
4. diff /tmp .emacs-custom.el #obtain the code snippet.
5. mv /tmp/.emacs-custom ~
6. restart emacs
Just to obtain the code snippet!

Let's compare
https://overpass-turbo.eu/s/177G
As you see, just like RMS wants:
code produced shown to the user,
and not need six whole steps to
"dig it back out and have a look".