sidebar: unify scroll event handling with header logic by using only one event handler, handle long sidebars by stopping at container end (needs some more refinement), pause and revert sidebar scrolling when a tour starts, re #4525, re #3768

git-svn-id: svn://develop.studip.de/studip/trunk@31491 c5b85792-5220-0410-ad23-ed5b6b8ad999