Separate menu placement logic from menu primitive

[jossmac]

Apologies for all the white-space changes; prettier had a field day.

Basically this moves the logic for placing and resizing the menu into MenuPlacer, which wraps the Menu primitive (now actually a primitive).

Also, menu placement logic now checks the explicit margin value on the menu element rather than pulling the menuGutter value from theme.

Why?

• separation of concerns
• menu primitive can be easily replaced/augmented by consumers [v2]How do I add a menubuffer by version2 ?  #2965
• max-height constraints weren't being passed on to the MenuList, which was breaking position fixed behaviour when close to the bottom of the viewport

Next

• needs a bit of cleanup
• it's now basically a half implemented react-popper substitute; might be worth revisiting

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Resolves #2965

[gwyneplaine]

Just needs an example and fixed tests, otherwise LGTM