Buttons for adding nucleons can get stuck in disabled state when shuffling listeners

[jbphet]

This was found during the "Does the sim behave correctly when listener order is shuffled?" step of the code review, see #165.

When testing with randomized listener order, the buttons for adding protons, neutrons, and both can be left in a disabled state after a reset.

To duplicate:

1. Load the sim with the random seed 423809 (I saw this using several seeds, this is just one of them) and go into the "Decay" screen. The URL I used on my system was http://localhost:8080/build-a-nucleus/build-a-nucleus_en.html?brand=phet&ea&listenerOrder=random(423809).
2. In quick succession, add two protons and then press the reset button
3. All buttons for adding nucleons end up disabled, (see screenshot below), making the screen basically unusable

[zepumph]

Fixed over in #105. Thanks for the report. The bug was actually over in axon, where observableArray needed to lock in its lengthProperty listeners when it was marked as having order dependencies phetsims/axon@4f571cb