Customize: Introduce starter content and site freshness state.

A theme can opt-in for tailored starter content to apply to the customizer when previewing the theme on a fresh install, when `fresh_site` is at its initial `1` value. Starter content is staged in the customizer and does not go live unless the changes are published. Initial starter content is added to Twenty Seventeen.

* The `fresh_site` flag is cleared when a published post or page is saved, when widgets are modified, or when the customizer state is saved.
* Starter content is registered via `starter-content` theme support, where the argument is an array containing `widgets`, `posts`, `nav_menus`, `options`, and `theme_mods`. Posts/pages in starter content are created with the `auto-draft` status, re-using the page/post stubs feature added to nav menus and the static front page controls.
* A `get_theme_starter_content` filter allows for plugins to extend a theme's starter content.
* Starter content in themes can/should re-use existing starter content items in core by using named placeholders.
* Import theme starter content into customized state when fresh site.
* Prevent original_title differences from causing refreshes if title is present.
* Ensure nav menu item url is set according to object when previewing.
* Make sure initial saved state is false if there are dirty settings without an existing changeset.
* Ensure dirty settings are cleaned upon changeset publishing.

Props helen, westonruter, ocean90.
Fixes #38114, #38533.


git-svn-id: https://develop.svn.wordpress.org/trunk@38991 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/customize/class-wp-customize-nav-menu-item-setting.php

@@ -602,6 +602,17 @@ class WP_Customize_Nav_Menu_Item_Setting extends WP_Customize_Setting {
 			}
 		}
 
+		// Ensure nav menu item URL is set according to linked object.
+		if ( ! empty( $post->object_id ) ) {
+			if ( 'post_type' === $post->type ) {
+				$post->url = get_permalink( $post->object_id );
+			} elseif ( 'post_type_archive' === $post->type && ! empty( $post->object ) ) {
+				$post->url = get_post_type_archive_link( $post->object );
+			} elseif ( 'taxonomy' == $post->type && ! empty( $post->object ) ) {
+				$post->url = get_term_link( (int) $post->object, $post->object );
+			}
+		}
+
 		/** This filter is documented in wp-includes/nav-menu.php */
 		$post->attr_title = apply_filters( 'nav_menu_attr_title', $post->attr_title );