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


Built from https://develop.svn.wordpress.org/trunk@26141


git-svn-id: http://core.svn.wordpress.org/trunk@26052 1a063a9b-81f0-0310-95a4-ce76da25c4cd