Plupload wrapper: Call init callback on PostInit event.
Fixes the background image uploader in Customizer, which was broken after [27316].

fixes #27620.
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TinyMCE: improve setting disabled/enabled/activated states on the Unlink button, fixes #27309
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Database: Fall back from ext/mysqli to ext/mysql if the connection fails.

This allows us to avoid breaking a site that works under ext/mysql but is misconfigured for ext/mysqli.

props pento.
see #21663.

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Themes: Hide 'Add New' with no JS. see #27055.

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Theme Installer: Fix sorting, counts, keyboard navigation; add prev/next to previews.

props matveb.
see #27055.

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Fix typo in verify_file_md5() description. see [25541].

props siobhan.
see #27651.
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Clarify tooltips on Edit Image screen.

props siobhan.
fixes #27651.
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