Improve withConstrainedTabbing to take into account click on the content

[afercia]

Splitting this out from #9973

While testing the Modal component, an edge case surfaced which should be taken into account by withConstrainedTabbing. To reproduce:

• test with Safari, Edge, or IE 11 where this edge case is more evident
• open the Help modal (on a mac, press Control-Option-H, on Windows: Shift-Alt-H)
• press Tab and verify focus stays on the Close button
• click in the middle of the content
• press Tab
• instead of staying within the modal, focus is now on the document root

Testing the modal, it was pointed out that adding a tabIndex="-1" to the modal wrapper could help. It does help but if still fails on Safari, Edge, and IE11. Instead of relying on browsers implementation of focus and tab starting point, a better option would trying to address this issue programmatically.

Proposed solution

• add a tabIndex="-1" to the withConstrainedTabbing wrapper instead of to the single component wrapped in withConstrainedTabbing
• account for the case when the keydown event target is the wrapper itself, which happens when clicking in the content and then pressing the Tab key
• in this case, move focus to the first tabbable element