factor out duplicated code for keypad?

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Related to code review #103, these 2 items:

• Are any significant chunks of code duplicated? This will be checked manually as well as with https://github.com/danielstjules/jsinspect or grunt find-duplicates
• Is there anything that should be generalized and migrated to common code?

KeypadLayer and KeypadPanel were copied from unit-rates, then modified slightly. I recognize them because I did the implementation in unit-rates, and I'm shown as @author.

Should something be generalized and moved to common code?

Worth noting is that the unit-rates implementation is sub-optimal. I would have preferred to have used joist.Dialog instead of creating my own "keypad layer". But Dialog had (and still has) serious problems.

This issue should not prevent publication to this sim, can be addressed later.

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After thinking about this... I'm reluctant to factor out common code because this implementation would be unnecessary if https://github.com/phetsims/joist/issues/359, phetsims/scenery-phet#283 and phetsims/scenery-phet#271 were addressed. So I think that a better idea would be to create issue in projectile-motion and unit-rates indicating that KeypadLayer + KeypadPanel should be replaced when those issues are addressed.

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I created #127 in projectile-motion. And added a comment to the existing issue phetsims/unit-rates#153 in unit-rates.

Closing this issue.