This adds the ability for themes to add support for videos in custom headers
by passing `'video' => true` as an argument when adding theme support for
custom headers.
Custom video headers are managed through the “Header Visuals” (i.e. “Header Image”)
panel in the Customizer where you can select a video from the media library or set a
URL to an external video (YouTube for now) for use in custom headers.
This introduces several new functions:
`has_header_video()` – Check whether a header video is set or not.
`get_header_video_url()` – Retrieve header video URL for custom header.
`the_header_video_url()` – Display header video URL.
`get_header_video_settings()` – Retrieve header video settings.
`has_custom_header()` – Check whether a custom header is set or not.
`get_custom_header_markup()` – Retrieve the markup for a custom header.
`the_custom_header_markup()` – Print the markup for a custom header.
And a new file, `wp-includes/js/wp-custom-header.js` that handles loading videos
in custom headers.
This also enables video headers in the Twenty Seventeen and Twenty Fourteen themes.
Props davidakennedy, celloexpressions, bradyvercher, laurelfulford, joemcgill.
Fixes#38172.
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* Introduces `customize-selective-refresh-widgets` theme support feature and adds to themes.
* Introduces `customize_selective_refresh` arg for `WP_Widget::$widget_options` and adds to all core widgets.
* Remove `selective_refresh` from being a component that can be removed via `customize_loaded_components` filter.
* Add `WP_Customize_Widgets::get_selective_refreshable_widgets()` and `WP_Customize_Widgets::is_widget_selective_refreshable()`.
* Fix default `selector` for `Partial` instances.
* Implement and improve Masronry sidebar refresh logic in Twenty Thirteen and Twenty Fourteen, including preservation of initial widget position after refresh.
* Re-initialize ME.js when refreshing `Twenty_Fourteen_Ephemera_Widget`.
See #27355.
Fixes#35855.
Built from https://develop.svn.wordpress.org/trunk@37040
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* Enforce `===` instead of `==`
* Fix confusing use of `!`
* Remove unused variables like `calcNext`, defined but never used
* Proper use of braces
* Move functions out of if/else statements
* Document unknown functions as Microsoft-specific
Built from https://develop.svn.wordpress.org/trunk@25995
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