Fixed loss of focus when clicking the Copy button

[ffissore]

When the Copy button is pressed, focus is lost and scrolling the page with the arrow keys stops working. Fixed by giving focus back to the trigger object (clipboard.js provided) which is the copy button itself

[ffissore]

Steps to reproduce:

1. open the example book, open a page with a code block
2. try scrolling using your keyboard arrow keys, verify it works
3. click on any "copy" button on the right side of a code block
4. try again to scroll using the arrow keys, verify it's not working any more

[ehuss]

This seems to work.

An alternative is to update the version of clipboard.js to the latest (2.0.4) which already has this fix (zenorocha/clipboard.js#419). AFAICT, the only change necessary to switch to 2.0 is to use the constructor ClipboardJS instead of Clipboard (details are in https://github.com/zenorocha/clipboard.js/releases).

[ffissore]

Thanks for the hint. Delegating the fix to ClipboardJS makes much more sense. I've updated the PR accordingly

[Dylan-DPC-zz]

Thanks