POC: Undocking of tiles into their own viewports

[emilk]

• Experimentation for https://github.com/rerun-io/egui_tiles/issues/30

Try it with cargo run --example advanced

Supports:

• [x] Undocking a tile by dragging it out of the root viewport
• [ ] Drag-dropping within a child viewport
• [ ] Drag-dropping between viewports
• [ ] Re-docking