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.



git-svn-id: https://develop.svn.wordpress.org/trunk@26140 602fd350-edb4-49c9-b593-d223f7449a82