Fix misplaced Popover when using Floating UI (#566)

[atmelmicro]

Closes #566

[mbohal]

But we are open to every implementation user decides to use.

I agree with @bedrich-schindler that we do not want to embrace Floating UI to closely.

I think the question is if some other placement implementation is likely to express position differently then x, y and position. I don't feel versed enough in CSS to decide this. I would think it less likely, but I'm don't really know.

I can also imagine that we would require to pass either placement or placementStyle, so that only one can be set at the time. But this is just question arising in my head.

So you mean to have one new prop:

placement: PropTypes.shape({
    /**
   * This sets the CSS property `position` of the popover. This is reserved for use with Floating UI.
   */
  position: PropTypes.string,
  /**
   * This sets the CSS property `top` of the popover. This is reserved for use with Floating UI.
   */
  x: PropTypes.string,
  /**
   * This sets the CSS property `left` of the popover. This is reserved for use with Floating UI.
   */
  y: PropTypes.string,
})

so we can then easily switch between placement formats?