Add support to export tasks to calendar

**Goal**
- As a user
- I want to see time related tasks in my calendar
- so that I see them in parallel to items in my calendar

**Suggested approach**
In order to achieve this goal I suggest to have a button per area/project (or regular export on start)

**Additional context**
 - partly implemented at https://github.com/AlexanderWillner/things.sh
