Remainder of fix for FM-383: Ensure that when SerialElement (via TraitLoader) attempts to load the next playable child (on child transitions), it takes into account the possibility that the LoadTrait of a child that it's loading will be replaced by another LoadTrait (which would then become the one it needs to load). Also updated the CaptioningProxyElement to use a higher listener priority so that it ensures that the SerialElement doesn't think that the load is complete when transitioning from child to child. (This is a necessary step for clients who use the swapping LoadTrait trick, but there might be a more robust approach to the same problem.)
