Migrate from egui_Tile::TileId to proper blueprint IDs in ViewportBlueprint API

[abey79]

What

The PR removes many instances of egui_tiles::TileId in the Viewport/ViewportBlueprint/SpaceViewBlueprint/ContainerBlueprint APIs, replacing them by SpaceViewId and ContainerId as appropriate. It also adds some new APIs to support operations previously performed via the tile tree.

This PR also removes the legacy_container_blueprint feature flag (was added for 0.12 and has been defaulting to false for a while).

Note that much of the actual data mutation is still done via tile tree and synchronised back to the blueprint store. This should be further improved in the future.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
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• 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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[jleibs]

👏 thank you! This really cleans up a lot.