Bring in the widgets component of MP6. See #25858.

* A cleaner UI, with a simpler design for available and inactive widgets, and more emphasis on your sidebars.
* Available widgets and sidebars expand to two columns at larger screen sizes making it easier to manage sites with lots of sidebars.
* New dropzone indicator for empty sidebars.

Props shaunandrews, joen, iammattthomas.


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Update the Themes screen, merging THX into core.

 * Name: THX38
 * Description: Update the Themes screen with a new design and experience.
 * Tags: visually-focused, bigger-screenshots, fast, mobile-friendly, backbone-driven
 * Author: matveb, shaunandrews, melchoyce, designsimply, shelob9
 * URI: http://wordpress.org/plugins/thx38/

fixes #25948


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