Clarify that unregister_default_headers() returns void in some situations. fixes #21876.

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git-svn-id: http://core.svn.wordpress.org/trunk@27418 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Heartbeat: Hooks should always receive unslashed data.

This affects the privileged hooks; the unprivileged hooks already received unslashed data.

props johnbillion, TobiasBg.
fixes #27260.

Built from https://develop.svn.wordpress.org/trunk@27576


git-svn-id: http://core.svn.wordpress.org/trunk@27419 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Style the cursor in the image editor in the media modal.

props tomauger.
fixes #27392.

Built from https://develop.svn.wordpress.org/trunk@27577


git-svn-id: http://core.svn.wordpress.org/trunk@27420 1a063a9b-81f0-0310-95a4-ce76da25c4cd
TinyMCE:
- Update the styling of the image caption and gallery toolbars.
- Fix issue where the hidden clipboard div in wpView.
- Switch to using the pencil dashicon for Edit.
- Add `max-width: 100%;` to images in the gallery preview and adjust column widths.
Props cramdesign, mattheu, gcorne, melchoyce, see #27320, fixes #27376, fixes #27354

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git-svn-id: http://core.svn.wordpress.org/trunk@27421 1a063a9b-81f0-0310-95a4-ce76da25c4cd