[Popover] Expose the component

[oliviertassinari]

Closes #6270 : Add some documentation and move the component to the public API
Closes #7573, #7426 : Block the scroll of the body, we are screwed when used inside a scrollable container that isn't the body. This is a limitation of the component. We would have to use react-popper to fix it.
Closes #7440 : Looks good now
Closes #7015 : Hops it's fine with this rewrite
Closes #7012 : We are no longer listening for the onWheel event
Closes #4439 : Hops it's fine with this rewrite

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• PR has tests / docs demo, and is linted.
• Commit and PR titles begin with [ComponentName], and are in imperative form: "[Component] Fix leaky abstraction".
• Description explains the issue / use-case resolved, and auto-closes the related issue(s) (http://tr.im/vFqem).