Add support for user-customizable icons

Add support for user-customizable icons

* doc/emacs/custom.texi (Specific Customization): Mention it.

* doc/emacs/display.texi (Icons): New node.

* doc/lispref/display.texi (Icons): New node.

* lisp/button.el (buttonize):
(button--properties, buttonize-region): Allow not overriding faces.

* lisp/cus-edit.el (custom-save-all): Save icons.
(custom-icon): New widget.
(custom-icon-value-create, custom-toggle-hide-icon)
(custom--icons-widget-value, custom-icon-set): Helper functions
for the widget.
(customize-icon): Main command.
(custom-icon-state-set, custom-icon-state): Helper functions.
(custom-theme-set-icons): Function to be used by theme writers.
(custom-set-icons): Function to be used in .emacs.
(custom-save-icons): New function.

* lisp/custom.el (custom-push-theme): Add icons.

* lisp/emacs-lisp/icons.el: New file.

* test/lisp/emacs-lisp/icons-tests.el: Add some tests.