dialogs: refine button handling (preserve accept and cancel classes), re #4404

git-svn-id: svn://develop.studip.de/studip/trunk@30697 c5b85792-5220-0410-ad23-ed5b6b8ad999
sidebar: update folder.php, re #3768

git-svn-id: svn://develop.studip.de/studip/trunk@30698 c5b85792-5220-0410-ad23-ed5b6b8ad999