Equation AccordionBox doesn't properly resize when interactiveEquationNode is partially hidden

[arouinfar]

For #180

When all three sliders in the Equation AccordionBox are hidden, the layout does not dynamically change to eliminate the blank space.
equationAccordionBox.interactiveEquationNode.*Slider.visibleProperty

Hiding the interactiveEquationNode resizes things as expected, however.

[arouinfar]

Actually, this appears to be a more general issue with the interactiveEquationNode which contains the sliders and an equationNode.

Hiding interatctivieEquationNode.equationNode also results in a gap in the layout.

[pixelzoom]

Reproduced in master.

[pixelzoom]

Fixed in the above commit. @arouinfar please review, close if OK.

[arouinfar]

Thanks @pixelzoom. Looks good in master, closing.