Convert Particles.changedCallbacks to use Emitter

[samreid]

Noted during #213.

Particles.js defines:

    this.changedCallbacks = []; // @protected called when the collection of particles changes

/// ...

    /**
     * Registers a callback that will be called when the collection of particles changes in some way
     * (eg, number of particles, particles move, ...)
     * @param {function} callback
     * @public
     */
    registerChangedCallback: function( callback ) {
      this.changedCallbacks.push( callback );
    }

Then Precipitate.js has this code:

    // @private Notify that the precipitate has changed.
    fireChanged: function() {
      var changedCallbacks = this.changedCallbacks.slice( 0 ); // copy to prevent concurrent modification
      for ( var i = 0; i < changedCallbacks.length; i++ ) {
        changedCallbacks[ i ]( this );
      }
    },

And ShakerParticles.js defines this code:

    // @private Notify that at least one particle was added, removed, or moved
    fireParticlesChanged: function() {
      var changedCallbacks = this.changedCallbacks.slice( 0 );
      for ( var i = 0; i < changedCallbacks.length; i++ ) {
        changedCallbacks[ i ]();
      }
    }

This code should be rewritten to use axon Emitter.

[pixelzoom]

This sim implements its own listener mechanism because (1) it was ported directly from the Java version, and (2) the port predates the existence of Emitter. I question the value (and cost) of such as small amount of working/straightforward code. But I guess it's good to be consistent.

[pixelzoom]

Same pattern noted in gravity-and-orbits and scenery, see above issues.

[pixelzoom]

Conversion to Emitter completed. Also required a change to TShakerParticles. Test in requirejs mode and with instance-proxies wrapper. Next savings was 11 lines of code.

@samreid please review.

[samreid]

The change set looks correct and I tested by saturating a solution then diluting it. Closing.