Customize: Add infrastructure for trashing/reverting of unpublished changes; introduce full-screen OverlayNotification for trashing and theme install/preview.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


git-svn-id: https://develop.svn.wordpress.org/trunk@41667 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/script-loader.php

@@ -571,6 +571,10 @@ function wp_default_scripts( &$scripts ) {
 		'expandSidebar'      => _x( 'Show Controls', 'label for hide controls button without length constraints' ),
 		'untitledBlogName'   => __( '(Untitled)' ),
 		'serverSaveError'    => __( 'Failed connecting to the server. Please try saving again.' ),
+		'themeDownloading'   => __( 'Downloading your new theme…' ),
+		'themePreviewWait'   => __( 'Setting up your live preview. This may take a bit.' ),
+		'revertingChanges'   => __( 'Reverting unpublished changes…' ),
+		'trashConfirm'       => __( 'Are you sure you would like to discard your unpublished changes?' ),
 		/* translators: %s: URL to the Customizer to load the autosaved version */
 		'autosaveNotice'     => __( 'There is a more recent autosave of your changes than the one you are previewing. <a href="%s">Restore the autosave</a>' ),
 		'videoHeaderNotice'  => __( 'This theme doesn\'t support video headers on this page. Navigate to the front page or another page that supports video headers.' ),