Texturize: Massive performance improvements (~600% faster); better handling of nbsp, double, and weird spaces; 136 new unit tests.

big props miqrogroove.
fixes #22692.

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Fix drag-and-drop upload in distraction-free writing.

props kovshenin.
fixes #27438.

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Media Manager: Hide 'Create Playlist' menu items until the user has uploaded audio or video.

props gcorne.
fixes #27554.

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Restyle the image editor so it looks way less hideous.

props avryl.
see #27544.

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Theme Installer: Remove unused variables and undesired commas.

see #27055.

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Add braces to wptexturize() after [27839]. see #22692.

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Theme Installer: Avoid race condition during pagination.

props matveb.
see #27055.

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Custom Headers: Simplify and consolidate the querying of custom headers for the customizer.

props mcsf.
see #21785.

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3.9-beta3

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Post beta3 version bump.

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