Add interactive highlights for sim-specific draggable objects.

[pixelzoom]

See interactive-highlights-quickstart-guide.md.

[pixelzoom]

@arouinfar ready for review, close if OK.

[arouinfar]

Thanks @pixelzoom.

The electromagnet and pickup coil appear to have an extra interactive highlight when mousing over the back portion of the coils, but otherwise things are looking good on main. I do not see the extra highlight for keyboard focus, just interactive highlights with the mouse.

[arouinfar]

Looks like the extra interactive highlight is indicative of bigger problems. I can move the back of the electromagnet coil with the mouse. I haven't been able to do the same for the pickup coil.

[pixelzoom]

Looks like the extra interactive highlight is indicative of bigger problems. I can move the back of the electromagnet coil with the mouse. I haven't been able to do the same for the pickup coil.

Yes indeed, see #56.

[pixelzoom]

I tried this change in CoilNode:

    this.backgroundNode = new FELMovableNode( coil, {
      hasKeyboardDragListener: false,
+     interactiveHighlightEnabled: false

... but then there is no interactive highlight for CoilNode when the pointer is over backgroundNode.

[pixelzoom]

@arouinfar and I discussed. The foreground and background are separate Nodes, and must be separate Nodes if the B-field and magnets are to "pass through" the coil. Rather than try to make the foreground interactive highlight appear when over the background, we're going to live with 2 interactive highlights. (We will have one focus rectangle, which is very doable and already implemented.