Finalize the Classical/Quantum selector on the "Photons" screen

[jbphet]

As part of the work for #65, a pair of radio buttons was added to allow the user to choose between classical and quantum modes for the photon behavior. It currently looks like this:

At today's design meeting I asked if we were definitely going to keep this around, and the answer was essentially "Probably, but we want to run it by the teachers who are consulting on this to make sure that they are okay with it". I figured we should have an issue to make sure we followed up on this.

Assigning to @arouinfar to start with.