tabs: clickable arrows in third navigation level, fixes #3662

git-svn-id: svn://develop.studip.de/studip/trunk@27406 c5b85792-5220-0410-ad23-ed5b6b8ad999
refine [27406], re #3662

git-svn-id: svn://develop.studip.de/studip/trunk@27407 c5b85792-5220-0410-ad23-ed5b6b8ad999