Whole play area in ChartIntroScreenView can be its own class

[Luisav1]

From #112. The whole play area including the node's below can be factored out into its own class.

• energy text and arrow
• protons and neutrons energyLevelNode
• dashed lines coming out of the mini-atom
• Maybe the nucleonNumberListener to update the mini-atom?

[Luisav1]

Decided with @zepumph that this change was not really worth it now that everything is working good (coordinate frames might change etc.). Closing.