Replace shouldReturnFocusAfterClose={boolean} with returnFocusTo={Element}

This must be an element that will gain focus after the modal
is closed.

The default behavior should be to return the focus to the element
on document.activeElement before open the modal, but...
in some browsers, this behavior is broken so we can't always
trust it.

So, the user must pass to this attribute an element
that must receive the focus later.