What

This PR wires the new "Add SV/Container" modal to the blueprint tree + button when the "Additive workflow" feature flag is enabled.

Now, the new SV or Container is added to the selected container if the selection consist of a single container. Otherwise the SV or Container is added to the root container.

• Closes: Use the new modal with the blueprint panel "+" button #4652
• Follow-up: The "Add SV/Container" modal should specify somewhere where the new item will be added #5011

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using newly built examples: app.rerun.io
  • Using examples from latest main build: app.rerun.io
  • Using full set of examples from nightly build: app.rerun.io
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG

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