Feat: Dock overlays to tracked devices.

[galister]

We need to come up with an intuitive way to set the parent device a given overlay.

Sticking overlays to devices is already handled in auto_movement.

All we need to do is set the OverlayState's spawn_point, spawn_rotation and relative_to.

Active overlays are stored in OverlayContainer, which is normally only accessible from the backend's main loop. However, overlay states can be manipulated from anywhere where AppState is available, by app.tasks.enqueue'ing a TaskType::Overlay. Example here.