dialog: add option to center to dialog's content horizontally and vertically, re #5156

git-svn-id: svn://develop.studip.de/studip/trunk@32804 c5b85792-5220-0410-ad23-ed5b6b8ad999
dialog: refine centering by utilizing box-sizing, refines [32804], re #5156

git-svn-id: svn://develop.studip.de/studip/trunk@32805 c5b85792-5220-0410-ad23-ed5b6b8ad999