Convert to one of the ZoomButtonGroup subclasses

[pixelzoom]

From phetsims/scenery-phet#652, this sim is a candidate for converting to one of the ZoomButtonGroup subclasses. The Buttons screen of the scenery-phet demo shows the 2 subclasses (screenshot below), PlusMinusZoomButtonGroup on the left, MagnifyingGlassZoomButtonGroup. If you don't want to change the look of the sim, use MagnifyingGlassZoomButtonGroup. If you think the more modern-looking PlusMinusZoomButtonGroup is more appropriate, consult with your sim designer.

[pixelzoom]

If there are questions about the model used by ZoomButtonGroup, these comments should be reviewed:

phetsims/scenery-phet#652 (comment)
phetsims/scenery-phet#652 (comment)
phetsims/scenery-phet#652 (comment)

[jonathanolson]

This is one of those sims where it (a) supports fractional zooms for phet-io and (b) uses the multiplier approach mentioned in phetsims/scenery-phet#652 (comment).

Since it's not currently compatible, I'm going to defer this (as suggested in phetsims/scenery-phet#652 (comment)).

[jonathanolson]

Common code will now allow this refactor.