Add integration of Things 3

For me Apple Reminders is just crap since Apple uses the new „sync“ engine.
Synchronisation on several devices works not well at all, takes hours for an update. Or even never some reminders get an update. No matter what app (Apple Reminder, BusyCal, …) I use.
Switched back to Things.
Now I start to find out how to “connect” Things AndAgenda …