default options need adjusting in ZoomButtonGroup hierarchy

[pixelzoom]

Related to #652. Something I noticed as I'm converting ph-scale to MagnifyingGlassZoomButtonGroup in phetsims/ph-scale#206...

ZoomButton uses buttonAppearanceStrategy: RectangularButton.ThreeDAppearanceStrategy (the default), while ZoomButtonGroup uses buttonAppearanceStrategy: RectangularButton.FlatAppearanceStrategy.

If we want to convert sims from ZoomButton to MagnifyingGlassZoomButtonGroup, and have them look the same as they do now, that means that every sim is going to have to set buttonAppearanceStrategy: RectangularButton.ThreeDAppearanceStrategy for MagnifyingGlassZoomButtonGroup. And we don't want to do that.

So I think the buttonAppearanceStrategyoverride should be moved from ZoomButtonGroup to PlusMinusZoomButtonGroup, so that MagnifyingGlassZoomButtonGroup uses 3D buttons. And if @samreid needs them to be flat in CCK:AC, then overridebuttonAppearanceStrategy` there.

[pixelzoom]

Ditto for cornerRadius. cornerRadius: 0 is specific to PlusMinusZoomButtonGroup, it doesn't belong in ZoomButtonGroup. Every sim ported to MagnifyingGlassZoomButtonGroup will be forced to set cornerRadius: 4 to keep the same look.

[pixelzoom]

Ditto for baseColor. MagnifyingGlassZoomButtonGroup should default to buttonOptions.baseColor: PhetColorScheme.BUTTON_YELLOW in order have the same look as ZoomButton. Otherwise every converted sim will need to set baseColor.

[pixelzoom]

Ditto for magnifyingGlassOptions.glassRadius. MagnifyingGlassZoomButtonGroup uses 10, ZoomButton uses 15. I'm guessing the values here were set for CCK:AC. They need to be set to match ZoomButton, or converting sims from ZoomButton to MagnifyingGlassZoomButtonGroup is going to be a pain.

[pixelzoom]

In the above commits, I've adjusted the default options so that:

• PlusMinusZoomButtonGroup has buttons that are flat, white, square corners.
• MagnifyingGlassButtonGroup has buttons that default to the look of ZoomButton (3D, yellow, rounded corners,...) This makes it easier for sims to migrate from ZoomButton to MagnifyingGlassButtonGroup.
• CCK:AC overrides defaults for MagnifyingGlassButtonGroup to provide the "look" that's desired in that sim.

@samreid please review.

[samreid]

I reviewed the commits and tested the default components in the scenery-phet harness, and tested the buttons in CCK-AC. Everything looks good to me, thanks! Closing.