Menu & DropdownMenu: Add horizontal orientation

[sanbornhilland]

Ref #2001

Description

Introduces horizontal orientation to the core Menu component and the DropdownMenu. This is a WIP as I'm not sure all the relevant changes that I need to make to address #2001. I'd like to get some input if this is headed in the right direction before committing more time and adding tests. Once we've addressed DropdownMenu I believe we can add the same support for Select.