Instrument ElementFollower for phet-io

[jbphet]

#388 describes some problems, as well as a "workaround-ish" solution to problems where the thermometers don't always manifest the correct element-following behavior in the state wrapper. The solution that was used to solve the problem involved monitoring the isSettingPhetioStateProperty property and doing some very specific state updates at the beginning and end of state settings. @zepumph and I discussed this, and we agreed that it would be way better to instrument ElementFollower so that its state is included in part of the state setting operation.

@zepumph said he was up for giving this a try. As of this moment, we are only planning to do this on master, since we have what appears for now to be a working solution on the 1.4 branch, and publication from that branch is imminent.