Migrate ical4j 3.x → 4.x

[rfc2822]

ical4j 4.0 will be released soon, and new development will then only be available in 4.x.

• Wait until ical4j 4.x is released and stable
• Create branch that uses ical4j 4.x
• Make sure that everything is working (including tests)

Update: Because this is such a large change, we should do it with extreme care (as it has the potential to break DAVx5 in many ways), extensive testing and when it's time for it.

[ArnyminerZ]

Some annotations regarding the migration (updating):

• Uses of obj is DateTime have been replaced with until.isSupported(ChronoUnit.HOURS), which should work fine as an alternative. Maybe it's not a bad idea to use DateUtils.isDateTime or create a shortcut of something like DateUtils.hasTime. This is because the new version uses Temporal which may be an Instant, a ZonedDateTime, or a LocalDate, to give some examples. So we can't use types here.

[rfc2822]

Closing for when it's time :)