Twenty Fourteen: add visual divider lines to child list items in widgets. Props iamtakashi, fixes #26415.

git-svn-id: https://develop.svn.wordpress.org/trunk@26642 602fd350-edb4-49c9-b593-d223f7449a82
Twenty Fourteen: minor code cleanup in Customizer file, props obenland. Fixes #26414.

git-svn-id: https://develop.svn.wordpress.org/trunk@26643 602fd350-edb4-49c9-b593-d223f7449a82