Convert symbol prettification into minor mode and global minor mode.

Convert symbol prettification into minor mode and global minor mode.

* etc/NEWS: Document new Prettify Symbols mode.

* lisp/progmodes/prog-mode.el (prettify-symbols-alist): Rename from
  `prog-prettify-symbols', and make a local defvar instead of defcustom.
  (prettify-symbols--keywords): Rename from `prog-prettify-symbols-alist'
  and make a local defvar.
  (prettify-symbols--compose-symbol): Rename from
  `prog--prettify-font-lock-compose-symbol'.
  (prettify-symbols--make-keywords): Rename from
  `prog-prettify-font-lock-symbols-keywords' and simplify.
  (prog-prettify-install): Remove.
  (prettify-symbols-mode): New minor mode, based on `prog-prettify-install'.
  (turn-on-prettify-symbols-mode): New function.
  (global-prettify-symbols-mode): New globalized minor mode.

* lisp/emacs-lisp/lisp-mode.el (lisp-mode-variables):
* lisp/progmodes/cfengine.el (cfengine3-mode):
* lisp/progmodes/perl-mode.el (perl-mode): Don't call `prog-prettify-install';
  set `prettify-symbols-alist' instead.