Keyboard navigation friendliness for themes.php.

Merges [26922] to the 3.8 branch.

props matveb, azaozz, jorbin.
fixes #26527.

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Themes: Search the theme's slug too.

Merges [26921] to the 3.8 branch.

props matveb.
see #26730.

Built from https://develop.svn.wordpress.org/branches/3.8@26957


git-svn-id: http://core.svn.wordpress.org/branches/3.8@26836 1a063a9b-81f0-0310-95a4-ce76da25c4cd