Drag bug with electromagnet coil "background layer"

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The background layer of ElectromagnetNode's CoilNode detaches from the coil when dragging it:

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This problem exposes some weirdness in the class hierarchy. PickupCoil extends Coil, and has LightBulb and Voltmeter fields. Electromagnet extends CoilMagnet, and has fields SourceCoil, DCPowerSupply, and ACPowerSupply. And SourceCoil has its own positionProperty, which is what is being set when dragging the background layer of the coil. This is also a problem for PhET-iO, because we have both electromagnet.positionProperty and electromagnet.sourceCoil.positionProperty.

I'm thinking that positionProperty should be removed from Coil, and PickupCoil should be structured more like Electromagnet.

Or maybe there should be another class (PickupDevice?) that include the PickupCoil, LightBulb and Voltmeter?

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This was addressed in the above commits. Rather than have PickupCoil extends Coil, I made PickupCoil have a Coil as a subcomponent. This allows me to get rid of positionProperty in Coil, which is what is causing the problem with the electromagnet background being draggable separately.

I don't feel like it needs a review, so closing.