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

[rfc2822]

After everything has been rewritten in the working branch.

• Make sure there are no left TODOs
• Check desugaring with Android 7 (it's about the TimeZone.toZoneIdCompat method – is it still needed / may we already use TimeZone.toZoneId? Otherwise replace everything. Check DAVx5 sync with Android 7 when ical4j 4.x is merged.)