fix(Popup): restore ability to render popup from React element

[kuzhelov]

TODO

• update changelog

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With recent changes being introduced to shorthand factories (specifically, because of the change that now dictates to pass React Element shorthand value 'as is'), the following way of defining Popup has become broken:

<Popup content={<p>This is the content that will be rendered 'as is'</p>} ... />

The way it was broken is that now the content of the <p> element is, essentially, not visible (however, while still rendered in the DOM tree), because positioning styles are not applied.

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This corresponds to the following example on the docs page:

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This PR restores this functionality.

[layershifter]

This issue shows that our idea from #496 may be not so good.

The factories change is not released yet, so I will block this PR for additional discussion on sync.

[layershifter]

Unblocked, let's go.

[kuzhelov]

@layershifter

As we've agreed with @layershifter while having discussion around that, moving with this now but leaving this topic for future discussion around shorthand factories, as this change might serve as a warning sign for the approach we've taken, especially if this pattern

isValidElement() ? clone() : create()   // © @layershifter :)

will be repeated for other components in many places.