RNG-372 Fix focus lock for web components 🐐

Fix an issue with the focus locking being too greedy in the <Panel> when it is wrapped in a web component.

The issue was reported and addressed in a recent update of the underlying react-focus-lock library used: https://github.com/theKashey/react-focus-lock/issues/188
Updated the <Overlay> (used by the <Panel>) as well as the <Modal> and <Takeover>.
The update introduced a new bug however: the close button in the header would be excluded from the tab sequence if it was implemented as a <Button.Close isSemantic={false}> (ie <span role="button" tabindex="0">). To compensate, the default value for the isSemantic prop in the headers of all these components is now true (with an isCloseButtonSemantic prop added to the <Panel.Header> and <Modal.Header> in case this needs to be reverted back to false in some cases where global CSS causes painful conflicts.
The refHeading prop for <Panel.Header> was no longer being used and has now been removed. This was not flagged as a major version bump since that prop actually had no effect on anything.
The "return focus" functionality of the react-focus-lock library does not seem to be working when the component is wrapped in a web component, so this behaviour will need to be applied manually in these cases, and has been included in the "Web Component" storybook example.

acl-services / paprika