Use buttons

Note: this won't compile until we release o-ft-buttons 1.8.0.

Nb: this PR is to merge into the `mixins` branch, not master.
`tabindex` > 0 is considered harmful, so I'm not sure how to do what you suggest.

If the outline looks strange, developers can remove it on the first item (as long as it's not on an interactive element).

Looking forward to having feedback from implementors on this feature, as accessibility is one of Origami's priorities.