Fix the unit tests for Imagick alpha.

These tests were written to use the `assertImageAtAlpha()` helper method, whose
internals were specific to GD. As a result, the tests could not pass. This
changeset introduces GD- and Imagick-specific versions of this method.

Props voldemortensen.
Fixes #24871.
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Editor CSS: Add missing semicolons.

props cfoellmann.
fixes #30487.
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Update the Iris color picker to version 1.0.6.

Changes: https://github.com/Automattic/Iris/compare/1.0.4...1.0.6

props mattwiebe.
fixes #30365.
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git-svn-id: http://core.svn.wordpress.org/trunk@30540 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Customizer: Don't override `Section.isContextuallyActive()` in `SidebarSection`.

This fixes a bug where empty widget areas get deactivated in the Customizer.

fixes #30378.
see #30235.
props westonruter.
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Customizer: Only show the default button in the upload control if there is a default attachment.

props celloexpressions.
see #21483.
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Clear update caches when site language is changed.

The update screen has a re-install button which could show the wrong language locale after the site language was changed. That's because the update data is cached. 
Introduce `_wp_clear_update_cache()` as a helper function to clean up the update cache.

props SergeyBiryukov, ocean90.
fixes #30369.
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Improve the focus style for review links in the plugin info modal.

props johnbillion.
fixes #30375.
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git-svn-id: http://core.svn.wordpress.org/trunk@30544 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Remove incorrect documentation for `wp.media()`.

props ericlewis.
fixes #30090.
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git-svn-id: http://core.svn.wordpress.org/trunk@30545 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add basic unit tests for `is_plugin_active()` and `is_plugin_inactive()`.

Props voldemortensen.
Fixes #30489.
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TinyMCE: use the actual image node when calculating the position of the toolbar. Fixes a bug in old IE. See #30147.
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