Twenty Thirteen: better selector for widget titles. Fixes #23580.

git-svn-id: https://core.svn.wordpress.org/trunk@23490 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: improve styling for RSS widget title, fixes #23571.

git-svn-id: https://core.svn.wordpress.org/trunk@23491 1a063a9b-81f0-0310-95a4-ce76da25c4cd