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.