replace LevelSelectionItemNode with LevelSelectionButton

[pixelzoom]

LevelSelectionItemNode is deprecated per #59. It should be replaced by LevelSelectionButton in the sims listed below. This is not a straightforward conversion, since the API has changed significantly to support new features requested by designers.

• area-builder @jbphet
• arithmetic @jbphet
• balancing-act @jbphet
• build-an-atom @jbphet
• balancing-chemical-equations @pixelzoom
• expression-exchange @jbphet
• fraction-matcher @jonathanolson
• graphing-lines @pixelzoom
• make-a-ten @jonathanolson
• reactants-products-and-leftovers @pixelzoom

Last person to convert a sim:

• delete LevelSelectionItemNode

[pixelzoom]

My sims have been converted to LevelSelectionButton.

Assigning this to other developers who are responsible for sims in the check list above.

[pixelzoom]

By the way... One of the things that made this very easy to test for my sims was the ?showAnswers query parameter that I've implemented for all games. If other sims had this, I might have converted some of the sims that I'm not responsible for. Without, it's just too much time.

[pixelzoom]

Labeled for developer meeting to make developers aware of this, and confirm with @ariel-phet that this is properly labeled as "chip away".

And do we want to create an issue for each sim, or handle this here? An issue for each sim probably has a better chance of getting seen/addressed when it's time to revisit the sim.

[jbphet]

I have converted all sims for which I and the responsible developer.

[jonathanolson]

Refactored out usages, deleted. Closing.