Customize: Eliminate use of customize-loader in core so Customizer is opened consistently in top window.

* Open the door for future browser history feature in #28536, which is currently not feasible when customize-loader is used.
* Remove customizer-loader from being used on admin screens for Dashboard, Themes, non-shiny theme install/update.
* Keep the customize-loader functionality available for plugins, for the time being. It may become deprecated.
* Ensure `return` param in customizer links in Themes screen update to reflect `search` updated by `pushState`.
* Persist `return` when reloading Customizer due to theme switch, autosave restoration, or changeset trashing.
* Use `location.replace()` instead of changing `location.href` when trashing.
* Hide theme browser while Themes screen is loading when there is a `search` to prevent flash of unfiltered themes.
* Use throttling instead of debouncing when searching themes to ensure that screen is updated immediately on page load.
* Fix encoding and decoding of `search` param between URL and search field.
* Add support for dismissing autosaves when closing customize-loader, when it is used by plugins.
* Skip sending changeset UUID to customize-loader for population in browser location if changeset branching is not enabled.

See #28536.
Fixes #40254.


git-svn-id: https://develop.svn.wordpress.org/trunk@41797 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-customize-manager.php

@@ -4236,6 +4236,7 @@ final class WP_Customize_Manager {
 			),
 			'url'      => array(
 				'preview'       => esc_url_raw( $this->get_preview_url() ),
+				'return'        => esc_url_raw( $this->get_return_url() ),
 				'parent'        => esc_url_raw( admin_url() ),
 				'activated'     => esc_url_raw( home_url( '/' ) ),
 				'ajax'          => esc_url_raw( admin_url( 'admin-ajax.php', 'relative' ) ),