The ribbon enabled them from the history lengths alone, so while an undo
group was open on the document (an action spanning several calls) a click
ran edit.undo and got its refusal. The buttons now ask cmd::can_undo and
cmd::can_redo, the undo list is empty while a group is open, and the two
buttons carry a name and their enabled state for assistive technology. A UI
test opens a group, clicks both and checks the history and their state.
