Link system element's visibility to corresponding radio button's visibility

[chrisklus]

From 01/07/20 EFAC PhET-iO design meeting, for #216.

The current way to remove a system element (the tea kettle, for example) from one of the carousels in Studio is to make the tea kettle radio button visible: false, as well as the teaKettleNode itself visible: false so that you don't see it go by when selecting an element on either side of the tea kettle.

@arouinfar and @kathy-phet would prefer to not have to explain this to phet-io clients or make them deal with it, so they proposed linking the visibility of the system elements to the visibility of the radio buttons, so all clients will need to do is hide a given radio button, and the rest is taken care of. We should then make the visibleProperty of the system element nodes phetioReadOnly: true so that phet-io clients can't accidentally break this relationship.

[chrisklus]

I was able to do this without passing in all of the energy system nodes to EnergySystemElementSelector (where the visibility's are being linked), so this change worked out pretty cleanly. Closing.