PhET-iO review for 1.6 release

[arouinfar]

This was one of the earliest sims published with PhET-iO Studio, and I think it would benefit from another design review before the upcoming publication of 1.6.

Review will focus on:

• Review dynamic strings/layout behavior
• Verify stringProperties are linked appropriately in the view
• Review phetioFeatured elements, and update overrides as needed

[arouinfar]

I completed the design review and opened a few issues tagged above.

I also updated the phetioFeatured overrides in the above commit. Summary of the changes:

• Featured zoomLevelProperty to be consistent with other sims.
• Unfeatured massSlider.visibleProperty because clients should use the top-level visibleProperty instead.
• Found a few cases where a Property should have been featured for all bodies, but skipped over by accident once or twice (e.g. isCollidedProperty, velocityProperty).
• Featured measuringTape.endPointProperty & measuringTape.startPointProperty on the To Scale screen because clients may want to know the coordinates of the measuring tape and compare to the positionPropertyof the bodies in the Play Area.
• Featured formattedTimeProperty so clients have easier access to the time using the units in the view, such as Earth Days. (Model time is featured too, but it's always in seconds.)
• Featured timeControlNode.playPauseStepButtonGroup.visibleProperty to keep things consistent with timeControlNode.speedRadioButtonGroup.visibleProperty