RFC: Be able to define fallback order for Popup position

[WiNloSt]

The behavior of a Popup is if our desired position doesn't fit in the viewport, the component will calculate the next possible fallback position based on a predefined constant POSITIONS.

While this is convenient that users doesn't need to defined their own fallback position order. Some users, like me, might prefer having another fallback order for visual purpose. So, being able to override this POSITIONS variable somehow would benefit in this case.

I imagine if there would be something like this

<Popup
  trigger={<Icon name="heart" color="red" size="large" circular />}
  content="I am positioned to the top left"
  position="top left"
  fallbackPositions={["bottom left", "bottom center"]}
/>

What do you think?

[layershifter]

I have an idea to switch positioning of Popup to Popper.js after we will finish with refs (#2306).

In fact, current positioning is poorly tested and I don't want to add more complexivity there. However, I think that we will accept PR for current implementation.