Extend JS API with some functionality for managing segments

[philippotto]

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• execute the following in the console:

window.webknossos.apiReady(3).then(api => {
   const layerName = api.data.getSegmentationLayerNames()[0];
   api.tracing.registerSegment(1, [10, 10, 10], undefined, layerName);
   const groupId = api.tracing.createSegmentGroup("New group")

   api.tracing.updateSegment(1, {name: "myName", groupId}, layerName);
   const segment = api.tracing.getSegment(1, layerName);
   console.log(segment);
})

Issues:

• users reported missing some functionality

---

(Please delete unneeded items, merge only when none are left open)

• Updated changelog
• Updated migration guide if applicable
• Updated documentation if applicable
• Adapted wk-libs python client if relevant API parts change
• Removed dev-only changes like prints and application.conf edits
• Considered common edge cases
• Needs datastore update after deployment

[hotzenklotz]

What happens if the layer name does not exist?

[philippotto]

an error will be thrown which I find acceptable.