Adapt iCal4j recurrence

[mkroening]

I was starting a follow up on #49 to remove any Joda-Time dependency from this project, as "from Java SE 8 onwards, users are asked to migrate to java.time (JSR-310) - a core part of the JDK which replaces this project" (Joda-Time Home) and this library targets "JavaFX 8, 9, 10, and 11".

When looking at CalendarFXRecurrence, I wondered if the whole module could be replaced by the recurrence API of iCal4j. iCal4j implements RFC5545 (Supersedes RFC2445). It is already in the dependency tree as dependency of CalendarFXiCal and will adapt java.time (JSR-310) in the future (see ical4j/ical4j#242).

What do you think about that?

[dlemmermann]

That sounds like a very good idea. However I am not sure how big the impact will be on the public API of CalendarFX. If it is also possible to express the entire recurrence via a single string ("rrule" on Entry) then this would be great.

[mkroening]

Resolved by #51.