Support SFX + spatial

[Roms1383]

Add support for SFX, including spatialization which can be useful for e.g. devices.

Tracking emitters

Updating emitters should probably rely directly on EntityID instead of ref<GameObject>:

public class UpdateEmitterCallback extends DelayCallback {
  public let npc: wref<GameObject>;
  public func Call() -> Void {
     // ...
  }
}

It should also be more generic (NPCPuppet), independently of method(s): if it's still convenient, then no problem having same function with different signatures, or siblings functions.

[Roms1383]

Fixed by #23.