duplication in options to HSlider

[pixelzoom]

Related to #30 (code review).

In AtomPropertiesPanel, there's a lot of duplication in the options argument passed to the 2 HSliders.

Recommend to factor out the duplication, like this:

var sliderOptions = {
      trackFill: RSConstants.PANEL_SLIDER_FILL_COLOR,
      trackStroke: RSConstants.PANEL_SLIDER_FILL_COLOR,
      majorTickStroke: RSConstants.PANEL_SLIDER_FILL_COLOR,
      majorTickLength: 15,
      tickLabelSpacing: 2,
      trackSize: new Dimension2( sliderWidth, 1 ),
      thumbSize: RSConstants.PANEL_SLIDER_THUMB_DIMENSION,
      thumbCenterLineStroke: 'white',
      startDrag: function() { // called when the pointer is pressed
        model.userInteraction = true;
      },
      endDrag: function() { // called when the pointer is released
        model.userInteraction = false;
      }
    }

Then fill in the things that are specific to each slider, like this:

 var protonCountSlider = new HSlider( model.protonCountProperty, {
      min: RSConstants.MIN_PROTON_COUNT,
      max: RSConstants.MAX_PROTON_COUNT
    }, _.extend( {}, sliderOptions, {
      thumbFillEnabled: 'rgb(220, 58, 10)',
      thumbFillHighlighted: 'rgb(270, 108, 60)'
    } );

Similarly for neutronCountSlider.

Take special note of how _.extend is being used here (3 args), so that sliderOptions is not modified.

Note also that some of this duplication also occurs in AlphaParticlePropertiesPanel, but it's less of a concern. Up to you whether you want to address it there, and factor out a common set of options for all HSliders that you put in RSConstants.

[pixelzoom]

Looks good, closing.