Use Popover for Menu dropdowns

[acelaya]

Refactor Menu component so that it displays the dropdown inside a Popover component.

TODO

• Close popover when a menu item is clicked
• Close popover when clicking a submenu item, not its surrounding submenu.
• Make sure dropdown works when popover API is not supported (asNativePopover={false})

[acelaya]

This PR was a pre-requisite to simplify the DOM structure in the groups Menu, and allow long group names to be truncated.

However, that has been achieved reasonably via #7292 and #7289, so I'm closing this for now, as it presents new challenges.

We still probably want to migrate to Popover eventually, ideally when all browsers we support can use the native Popover API.