Add fade-out of sorts per scene, add optional duration-per-scene paramter to the URL

[hybridherbst]

Would allow doing something like

• https://InArmsReach.app?t=60 to have 60s per scene for a total of ~3 minutes for the entire experience

so that it's time-boxed for higher volumes of people in exhibition setups.

We'd still have to decide what the default is (no parameter).