Improve the UI for the entity query

[abey79]

What

This PR implements the minimal sets of UI touch up to make the new query editor releasable.
Note: both commits best reviewed separately (first one is pure refactor)

• Fixes Add/remove entity and query editor improvement #4962

Query DSL feature flag off

Query DSL feature flag on

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

• PR Build Summary
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[jleibs]

Thanks for splitting / reorganizing. Much easier to follow.