Added set active tree action by name to api and cross origin api

[MichaelBuessemeyer]

This PR adds an action to set a tree with a given name. active.

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• open a skeleton tracing and create some trees.
• Then use the following script in your console to check whether the selection works:
  window.webknossos.apiReady(3).then(async (api) => { api.tracing.setActiveTreeByName(<tree_name>)});

Issues:

• This PR has no issue.

---

• Updated changelog
• [ ] Updated migration guide if applicable
• [ ] Updated documentation if applicable
• [ ] Adapted wk-connect if datastore API changes
• [ ] Needs datastore update after deployment
• Ready for review

[philippotto]

Cool stuff! See my one nitpick before merging :)

[philippotto]

I think, this block can be shortened to:

const treeWithMatchingName = Object.values(trees).find(tree => tree.name == treeName)

(in case, flow doesn't get the type information object.values() might be replacable with _.values)

[MichaelBuessemeyer]

Thanks for this feedback/nagging about this 👍