checkbox components: example using custom width

[#45465385]

#interactives/basic-examples/pulldown-and-radio-buttons-with-custom-parameters.json

Note: there appears to be a problem with the way the
slider component is implementing custom width.

The "Velocity Vectors" and "KE Shading" checkbox widths
are set to 14em.

In order to get the sliders just above to line up I
had to set the width to 12em.