fix(Popup): Focus the last focused element outside Popup on ESC

[sophieH29]

This PR fixes the issue when a complex component is used as a trigger for Popup, but actually, the inner element of that component triggers the Popup. So when press ESC on Popup, the focus should go to the actual focusable element which triggered the Popup, not the whole component itself.

In the next example, MenuItem is a trigger for the Popup and this component has this structure

<li> - wrapper
   <a></a> // focusable element which actually triggers Popup
</li>

const items = [
  {
    key: 'editorials',
    content: 'Editorials',
  },
  {
    key: 'review',
    content: 'Reviews',
  },
  {
    key: 'events',
    content: 'Upcoming Events',
  },
].map(item => render => render(item, renderIten))

const renderIten = (MenuItem, props) =>
  <Popup
    content={{ content: <Button content="Click me" /> }}
    trigger={<MenuItem {...props}/>}
  />

const MenuExample = () => <Menu defaultActiveIndex={0} items={items} />

So when press ESC, MenuItem's wrapper <li> element will receive focus, instead of <a>, what breaks focus management on the page.

Closes #831

[codecov]

Codecov Report

Merging #861 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #861   +/-   ##
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  Coverage   93.54%   93.54%           
=======================================
  Files          21       21           
  Lines         728      728           
  Branches       69       69           
=======================================
  Hits          681      681           
  Misses         47       47

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[layershifter]

Do we need to introduce a new method for a single use?

[sophieH29]

I think it makes sense, don't want to mess up tryOpen method and still remain better reading