[ical4j 4.x] Final integration of ical4j 4.x

[rfc2822]

After everything has been rewritten in the working branch:

• Verify that all tests are active and succeed
• Make sure there are no left TODOs
• Remove obsolete code (like helpers)
• Check KDoc
• Check whether helpers could simplify the code base
• Check desugaring with Android 7 (TimeZone.toZoneIdCompat still needed / may we already use TimeZone.toZoneId? Otherwise replace everything. Check DAVx5 sync with Android 7 when ical4j 4.x is merged.)