Improve how the root container is displayed and handled in the blueprint tree

[abey79]

What

This PR changes how the root container is displayed and handled:

• It's labeled as "Viewport", since it defines the top-level organisation of the viewport.
• It's not collapsible, so it doesn't have a triangle. This reduces the right-ward drift.
• It (still) cannot be dragged (but now the difference with other containers is more obvious). It now behaves better when dragged over though.
• It cannot be removed.

Other than that, it still behaves mostly as other containers (in particular, it can be selected and edited).

Some changes were needed in the drag-and-drop code, making the "if-statement-of-death" incrementally more complicated, sadly.

• Fixes Present the root container as the top-level viewport container and differentiate it from other container #4909

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
• Docs preview
• Examples preview
• Recent benchmark results
• Wasm size tracking

[Wumpf]

I'm new to this code and I must say these are some really pretty ascii arts explaining me what's going on. Very newcomer friendly!

This design makes way more sense and execution looks solid!