Widget Customizer: Add a label to widget search.

props SergeyBiryukov.
fixes #27590.
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MEjs has been fixed upstream: https://github.com/johndyer/mediaelement/commit/1a75559c9d9ada344fee7a6f5f08ccc1e4b0fbfa
No more infinite loop.

This restores MediaElement to a clean build: `2.14.0`.

Fixes #27582. 
Thanks to jeremyfelt for tracking this down.


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Metadata for audio and video files:

* Make attachment metadata for audio files editable by providing a metabox on the Edit Media page
* Standardize on using the attachment title everywhere
* Label the Caption and Description fields for audio and video appropriately
* Make the playlist Underscore templates more straightforward

See #27574.


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In `wp_generate_attachment_metadata()`, when an audio or video files contains upload-able image bits in its ID3 tags, only upload it if the image has not already been uploaded. Determine this by checking for a `_cover_hash` value in post meta that matches the `md5` representation of the bits.

This prevents uploading an album of 10 songs and subsequently uploading 10 copies of the same album cover.

Props GregLone for the new filter/filter docs: `'attachment_thumbnail_args'`.
Fixes #27573.
 

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Cleanups for audio/video metadata, see [27862].

see #27574.

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Using percentages for playlist font-sizing was ambitious, but fell prey to `editor-style.css` in the admin. We have switched to using pixels. The editor styles were also forcing MediaElement players to inherit `font-family`, which was undesirable. And while we're at it, remove the top and bottom border from the playlist's list of tracks, which adds a dash of lovely to the playlist's default styles.

Props nacin for the addition of `&mdash;` in the playlist track list.
Fixes #27600.


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Use correct formatting function. see [27864], see #27574.

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Bump the MediaElement version in `script-loader.php` to `2.14.1`.

See [27861].


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Favor escaping over interpolation in Underscore templates for media. Translate some strings directly instead of passing via JS.

Props nacin.
See #27574.


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Cleanup up the display, escaping, and handling of ID3 data for media. Rename `wp_get_relevant_id3_keys()` to `wp_get_attachment_id3_keys()`.

Props nacin.
See #27574.


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Inline documentation for hooks in wp-includes/widgets.php.

Props eliorivero and Nessworthy for the initial patches. Props kpdesign.
Fixes #27129.

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