[ical4j 4.x] Start handling trigger period + test

We could start using the new ical4j 4.x provided functionality to take care of trigger periods + timezone handling.

https://github.com/bitfireAT/synctools/pull/215/changes#diff-927002d797deab5f90d226a9ebbcfc3f5c31782e14bbc26307dc595958273775R330

https://github.com/bitfireAT/synctools/pull/215/changes#diff-0517c7cfb6c6f02f483362e1c4a6b78f50d8530f63d8124344a45dae232f262dR350