Twenty Thirteen: use gallery shortcode filter for featured galleries, props obenland. Closes #23823.

git-svn-id: http://core.svn.wordpress.org/trunk@23782 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: remove `hover` styles for screen reader text.

git-svn-id: http://core.svn.wordpress.org/trunk@23783 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Update RTL styles for Customizer. see #23449.

git-svn-id: http://core.svn.wordpress.org/trunk@23784 1a063a9b-81f0-0310-95a4-ce76da25c4cd