Currently, there is a `TrackableObservableList` which is just a regular observable list with a listener attached to it which then calls an `onChanged` method. The users of this list then override this `onChanged` method to keep track of changes (this is more efficient than using a listener, but only IF you avoid creating the listener in the first place, like how its implemented for `ObjectBinding#onInvalidating`). In this case however, the listener is still created.

There are many such lists in use, multiple in each `Node` and many per stylesheet. All of these register a listener, which in turn creates an `ExpressionHelper` -- two instances that can be avoided.

The proposal is to remove `TrackableObservableList` and provide this functionality in `ObservableListBase` or one of its subclasses.