Review inputEnabledProperty

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phetsims/scenery#1158 changed pickableProperty to inputEnabledProperty. Review this sim's use of inputEnbledProperty.

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Review is done, and there were significant changes required. Here's a summary of where things are at.

There are 3 inputEnabledProperty elements in this sim.

In the Macro screen, input for the pH probe can be disabled using:

phScale.macroScreen.view.pHMeterNode.probeNode.inputEnabledProperty

In the My Solution screen, the H2O+ and OH- graph indicators are interactive. When input is disabled, their arrows are hidden. Input can be disabled using:

phScale.mySolutionScreen.view.graphNode.logarithmicGraphNode.indicatorH3ONode.inputEnabledProperty
phScale.mySolutionScreen.view.graphNode.logarithmicGraphNode.indicatorOHNode.inputEnabledProperty

Over to @arouinfar for review.

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I see that inputEnabledProperty was not added to the dropper, and @kathy-phet had mentioned that as a specific use case. Let's discuss that in #215.

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Reviewed changes with @arouinfar, and client guides look OK. Closing.