Checkbox cleanup

[pixelzoom]

Noticed while working on enabledProperty in another UI component.

• this.content and this.ownsEnabledProperty do not need to be instance fields. They are used only in the constructor. Convert them to local variables.

• this.enabledProperty has no visibility annotation. Typically @public in other UI components.

• Multiple lines are longer than 120 chars.

• Checkbox doesn't have a demo/test in the sun demo app.

• options checkboxAppearanceStrategy and contentAppearanceStrategy are unused. Other UI components have been fine with using opacity to indicate disabled, and other UI components typically have disabledOpacity. So switch to this in anticipation of boilerplate being replaced with a mixin or trait.

• Use SunConstants.DISABLE_OPACITY, rather than duplicating it.

[pixelzoom]

Transferring this issue from scenery-phet to sun, since Checkbox lives in sun.

[pixelzoom]

Work completed in the above commits. @chrisklus randomly selected to review.

[pixelzoom]

It's been a long time since this was assigned for review. @ariel-phet please prioritize.

[chrisklus]

Dang, sorry @pixelzoom. Review complete, changes look straightforward and good to go. Back to @pixelzoom to close.

[pixelzoom]

👍