Customize: Improve usability of Customize JS API.

* Eliminate need to pass both ID and instance in calls to `Values#add()` for panels, sections, controls, settings, partials, and notifications.
* Eliminate need to supply `content` param when constructing a `Control`.
* Unwrap the `options.params` object passed in constructors to just pass a flat `options`. (Back-compat is maintained.)
* Add support for `templateId` param for `Control` to override which template is used for the content.
* Remove unused `previewer` being supplied in `Control` instances.
* Rename `classes` to `containerClasses` on `Notification`.
* Automatically supply `instanceNumber` to improve stable sorting.
* Use `api.Notifications` for notifications in settings instead of `api.Value`.

See #30741.
Fixes #42083.

Built from https://develop.svn.wordpress.org/trunk@41726


git-svn-id: http://core.svn.wordpress.org/trunk@41560 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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

@@ -3604,7 +3604,7 @@ final class WP_Customize_Manager {
 
 		?>
 		<script type="text/html" id="tmpl-customize-notification">
-			<li class="notice notice-{{ data.type || 'info' }} {{ data.alt ? 'notice-alt' : '' }} {{ data.dismissible ? 'is-dismissible' : '' }} {{ data.classes || '' }}" data-code="{{ data.code }}" data-type="{{ data.type }}">
+			<li class="notice notice-{{ data.type || 'info' }} {{ data.alt ? 'notice-alt' : '' }} {{ data.dismissible ? 'is-dismissible' : '' }} {{ data.containerClasses || '' }}" data-code="{{ data.code }}" data-type="{{ data.type }}">
 				<div class="notification-message">{{{ data.message || data.code }}}</div>
 				<# if ( data.dismissible ) { #>
 					<button type="button" class="notice-dismiss"><span class="screen-reader-text"><?php _e( 'Dismiss' ); ?></span></button>