add aria label (integrated change from github - https://github.com/flameo/studip-bugbash-2013/commit/aa13d0f8e79e3201606dc82c46aca8f60ca6fd8e), re #3379

git-svn-id: svn://develop.studip.de/studip/trunk@26069 c5b85792-5220-0410-ad23-ed5b6b8ad999