Bug - Popper's not maintaining proper stacking z-index order

[mcoker]

Placeholder for the work from patternfly/patternfly#4973

I think we're going to need to look at:

• Should popper things be added/removed from the DOM as they're triggered? Or should they create a ref on the page, and re-use that ref where it is in the DOM (doesn't move) to show/hide popper thing? Or both?
  • Currently the modal creates a ref, but tooltips don't, s
• Updating popper to use more intentional z-indexes, instead of 9999 for everything - probably... It kinda works well the way it is now except for the bullet above. It's probably still a good to use a smarter z-index order for popper anyways.

[nicolethoen]

Given pending recommendations from core, relevant components should expose the z-index so it can be overridden by a consumer.