js: add generic caching lib, fixes #5205

git-svn-id: svn://develop.studip.de/studip/trunk@32889 c5b85792-5220-0410-ad23-ed5b6b8ad999
js cache: refine documentation, re #5205

git-svn-id: svn://develop.studip.de/studip/trunk@32890 c5b85792-5220-0410-ad23-ed5b6b8ad999
js cache: use undefined instead of null, re #5205

git-svn-id: svn://develop.studip.de/studip/trunk@32891 c5b85792-5220-0410-ad23-ed5b6b8ad999