[ical4j 4.x] Unify helpers / deduplicate#262

cketti merged 4 commits intoical4j-3to4bitfireAT/synctools:ical4j-3to4from

260-ical4j-4x-unify-helpers-deduplicatebitfireAT/synctools:260-ical4j-4x-unify-helpers-deduplicateCopy head branch name to clipboard