PF3 ExpandCollapse missing onToggle callback

[shaded-enmity]

The Expandable component in PF4 provides onToggle callback, however the PF3 ExpandCollapse component has no such callback, which makes it impossible to use when the expanded part triggers a re-render of a parent component.

I can submit a PR for this, let me know if such change is welcomed.

[rachael-phillips]

@shaded-enmity thanks for filing! Yes, it would be great if you could put up a PR for this one :)