Allow manually setting full span scopes

[abey79]

What

When #6491 migrated to using UiStack for full span scopes, it wasn't possible to manually set a full span scope somewhere. Because of that, a clip rect hack was used for the streams tree. With emilk/egui#4617, it's now possible to support that correctly, so this PR introduces UiExt::full_span_scope() and uses it for the streams tree.

This PR also bumps the egui/egui_commonmark/egui_tiles commits.

Checklist

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

Beautiful!

[emilk]

The PR number didn't get updated, but it got built: https://rerun.io/viewer/pr/6509?manifest_url=https://app.rerun.io/version/main/examples_manifest.json

and it works!