Revise the rulers implementation.

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A couple of problems with the current implementation of rulers:

(1) The rulers toolbox uses some elements of the Creator pattern, but it's not quite right. And it needs to be right in order to support (and possibly facilitate) alt input design for the toolbox, in #258.

(2) It creates a new RulerNode each time the scale changes (zoom in/out). This will be a problem for PhET-iO, where it is very-much preferred to have static elements (because dynamic elements are painful).

Relevant classes are:

• RulersToolbox
• GeometricOpticsRulersLayer
• GeometricOpticsRulerNode

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Lots of work on this today, including a chunk of PhET-iO instrumentation.

The relevant classes are now:

• RulersToolbox
• RulerIconNode
• GeometricOpticsRulerNode
• GeometricOpticsRuler

TODO:

• Convert GeometricOpticsRuler.positionProperty and GeometricOpticsRulerNode.dragBoundsProperty to model coordinates
• Figure out why we need zoomScaleProperty in GeometricOpticsScreenView. Can everything be done with zoomTransformProperty?

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I'm going to revise transforms in #277.

In summary, I used a modified version of the Creator pattern. It's really not creating anything, but is instead using 2 static instances of rulers. This works better for PhET-iO, where static instance are preferred.

Closing.