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.

Built from https://develop.svn.wordpress.org/trunk@41797


git-svn-id: http://core.svn.wordpress.org/trunk@41631 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/themes-rtl.css

@@ -7,6 +7,10 @@
   16.1 - Manage Themes
 ------------------------------------------------------------------------------*/
 
+body.js .theme-browser.search-loading {
+	display: none;
+}
+
 .theme-browser .themes {
 	clear: both;
 }

wp-admin/css/themes.css

@@ -7,6 +7,10 @@
   16.1 - Manage Themes
 ------------------------------------------------------------------------------*/
 
+body.js .theme-browser.search-loading {
+	display: none;
+}
+
 .theme-browser .themes {
 	clear: both;
 }