Customize: Allow users to more seamlessly create page-based nav menus during customization.

Introduces the ability to create stubs for the various post types to add to a given menu. This eliminates the need to leave the customizer to first create the post in the admin and then return to managing menus. Only the title of the newly-created post can be supplied; the post content will be blank and will need to be provided in the normal edit post screen outside the customizer, unless a plugin enables a post editing in the customizer experience. When a post is created and added to a nav menu in the customizer, the newly created post that is added to a menu is given the `auto-draft` status, and if the changes are not published, the `auto-draft` post will be automatically deleted within 7 days via `wp_delete_auto_drafts()`. However, if the customizer changes are saved, then these nav menu item `auto-draft` post stubs will be transitioned to `publish`.

Includes portions of code from the Customize Posts <https://github.com/xwp/wp-customize-posts> and Front-end Editor <https://github.com/iseulde/wp-front-end-editor> plugins.

For more information, see https://make.wordpress.org/core/2016/06/16/feature-proposal-content-authorship-in-menus-with-live-preview/

Props celloexpressions, westonruter, valendesigns, afercia, melchoyce, mapk, iseulde, mrahmadawais.
Fixes #34923.

Built from https://develop.svn.wordpress.org/trunk@38436


git-svn-id: http://core.svn.wordpress.org/trunk@38377 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-customize-setting.php

@@ -498,6 +498,8 @@ class WP_Customize_Setting {
 	/**
 	 * Fetch and sanitize the $_POST value for the setting.
 	 *
+	 * During a save request prior to save, post_value() provides the new value while value() does not.
+	 *
 	 * @since 3.4.0
 	 *
 	 * @param mixed $default A default value which is used as a fallback. Default is null.