Factor out airResistanceCheckbox and dragCoefficient readout to separate Node

[zepumph]

From #219 this grouping will help factor out code from the intro and vectors screen while achieving phet-io instrumentation goals.

[zepumph]

Review comment:

Group airResistanceCheckbox, airResistanceText, and dragCoefficientReadout into airResistanceControl or simply airResistance. (@kathy-phet do you have a preference for the name?)

[zepumph]

I factored out these components in the above commit. I ended up naming the new control AirResistanceControl so that is what I named the phet-io element as well. Let me know if you would like to see something else.

Furthermore I simplified some of the children tandem names. I think it is a bit clearer, but please let me know if you would like them to be something else.

[arouinfar]

@zepumph AirResistanceControl is great! One thing that is a little unclear is airResistanceControl.text. Could it be renamed title or titleNode (if it is indeed a Node)? Otherwise, everything looks good.

[zepumph]

I went with titleText, since it is a Text (a subtype of Node). This seemed better to me because in addition to getting the visibleProperty from Node, it also has a textProperty which comes from Text, In general, I have found that this suffix can help describe what component parts the PhET-iO element has. What do you think?

[arouinfar]

airResistanceControl.titleText.textProperty looks great, closing.