Townie helpers: Add generic animation for things appearing

[manuq]

Currently the townie helpers added in #2119 can:

• make objects appear (the needles at the Void quest "needles" level)
• make tiles change (fixing the bridge at the stealth level)
• make objects change (filling the inkwell at the third combat level).

It would be nice to add a generic animation when that happens. Idea: let the townie unravel and jump over the level to the position of things to change, emit some particles to cover the change, jump back and rebuild themselves. In the Void floor eater enemy we have something similar (particles covering the change).