Bring the theme browsing experience from 3.8 to the theme installer. First pass.

props matveb with assists from me and gcorne.
see #27055.

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A nice round commit number for a beta 1.

Built from https://develop.svn.wordpress.org/trunk@27500


git-svn-id: http://core.svn.wordpress.org/trunk@27342 1a063a9b-81f0-0310-95a4-ce76da25c4cd