Calibration: add floor plan

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Provide a "floorplan", e.g., for museum AR experiences: the map guides the user from task station to task station and from room to room, so they can discover the different action steps and (alternative) experiences.

A simple floor plan for the currently loaded experience is a good starting point - basically outlining the floor plane and showing where the calibration marker is and where the task stations are.

Superseded by this issue: #1246