Export Keypoints in COCO format

**Describe the solution you'd like**
It would be very useful if we could export keypoint annotations using the COCO keypoints [format](https://cocodataset.org/#format-data). At the moment, only segmentation and detection of COCO are supported afaik.

The additional "visibility" flag could be configured during setup of the project:
1. define the keypoint set(s)
2. define visibility behavior: E.g. consider missing entries as not-labeled (flag=0) and add a tick box to indicate that labeled instances are invisible (flag=1) with default flag=2 for each labeled keypoint.