Add 'onClose' event to Popover

[jerrebm]

It would be great to have some way of knowing when a Popover panel is closed. Here's an example use-case:

I would like to apply the filters when the popover is closed by clicking outside. I currently have no way of detecting this.

[paustint]

I hacked this together - here is the summarized version:

• parent can pass in onChanged to shared Popover component
• Use a custom ref to store open state, this ensures that no additional re-renders are needed when open state changes
• In the render function, call checkIfStateChanged which will set the ref, if modified
• If the parent provided the onChange prop, then we call it - we defer this to the nextTick in case the parent needs to make a state update based on the popover either opening or closing (which would is not allowed in the middle of a render)

Not the most ideal solution as the library should provide some events, but it works very well.

    const isOpen = useRef<boolean>(false);


    const checkIfStateChanged = useCallback(
      (open: boolean) => {
        if (open !== isOpen.current) {
          isOpen.current = open;
          if (onChange) {
            nextTick(() => onChange(open));
          }
        }
      },
      [onChange]
    );

    return (
      <HeadlessPopover className="slds-is-relative" as="span">
        {({ open }) => {
          checkIfStateChanged(open);
          return (