popup: remove wrapper?

[wkeese]

As mentioned in #266, consider removing popup's wrapper node completely, and just position the popup itself.

That would also resolve a weirdness where both the popup and popup wrapper have either aria-label or aria-labelledby defined, which seems redundant. It also (currently) doesn't work well when the popup defines aria-labelledby instead of aria-label: the popup wrapper still defines aria-label.

Note: Currently we need to keep the wrapper to support IE11, due to flexbox bugs when the popup is display:flex. The workaround is to encase the popup in another display:flex node.