Customize: Add infrastructure for trashing/reverting of unpublished changes; introduce full-screen OverlayNotification for trashing and theme install/preview.

* Introduce a new `wp.customize.previewer.trash()` JS API to trash the current changeset, along with logic to `WP_Customize_Manager` to handle deleting changeset drafts.
* Add `trashing` to `wp.customize.state` which is then used to update the UI.
* UI for trashing is pending design feedback. One possibility is to add a new trash button to Publish Settings section that invokes `wp.customize.previewer.trash()`.
* Improve logic for managing the visibility and disabled states for publish buttons.
* Prevent attempting `requestChangesetUpdate` while processing and bump processing while doing `save`.
* Update `changeset_date` state only if sent in save response.
* Merge `ThemesSection#loadThemePreview()` into `ThemesPanel#loadThemePreview()`.
* Remove unused `autosaved` state.
* Start autosaving and prompting at beforeunload after a change first happens. This is key for theme previews since even if a user did not make any changes, there were still dirty settings which would get stored in an auto-draft unexpectedly.
* Allow `Notification` to accept additional `classes` to be added to `container`.
* Introduce `OverlayNotification` and use for theme installing, previewing, and trashing. Such overlay notifications take over the entire window.

Props westonruter, celloexpressions.
See #37661, #39896, #21666, #35210.

Built from https://develop.svn.wordpress.org/trunk@41667


git-svn-id: http://core.svn.wordpress.org/trunk@41501 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/js/customize-base.js

@@ -809,6 +809,14 @@ window.wp = window.wp || {};
 		 */
 		templateId: 'customize-notification',
 
+		/**
+		 * Additional class names to add to the notification container.
+		 *
+		 * @since 4.9.0
+		 * @var {string}
+		 */
+		classes: '',
+
 		/**
 		 * Initialize notification.
 		 *
@@ -821,6 +829,7 @@ window.wp = window.wp || {};
 		 * @param {string}   [params.setting] - Related setting ID.
 		 * @param {Function} [params.template] - Function for rendering template. If not provided, this will come from templateId.
 		 * @param {string}   [params.templateId] - ID for template to render the notification.
+		 * @param {string}   [params.classes] - Additional class names to add to the notification container.
 		 * @param {boolean}  [params.dismissible] - Whether the notification can be dismissed.
 		 */
 		initialize: function( code, params ) {
@@ -834,7 +843,8 @@ window.wp = window.wp || {};
 					data: null,
 					setting: null,
 					template: null,
-					dismissible: false
+					dismissible: false,
+					classes: ''
 				},
 				params
 			);