Please cleanup your TODO item.

Overall, I initially found this code somewhat confusing, which says something to its unintuitivity (though, it still might be acceptable).  You may or may not find [this paper](http://lampwww.epfl.ch/~imaier/pub/DeprecatingObserversTR2010.pdf) useful in avoiding the Observer Pattern.

At the very list, at least make `updateDisplayerListeners` and `requestDisplayUpdateListeners` private.