* 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
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