There were no usages of more than one column being provided per row, so
it seemed like unnecessarily complexity. I'm currently trying to reduce
complexity so we can improve the layout of the results screen, which
currently has up to three levels of nested `GridContainer`s.

Of note, I can't add backwards compatibility because the method
signature has not changed in `Ruleset` (only the return type). If we do
want to keep compatibility with other rulesets, we could designate a new
name for the updated method.

`dimensions` would always receive exactly one item, so might as well
inline it.

And yes, at this point the grid container is mostly a glorified
`FillFlowContainer { Direction = FlowDirection.Vertical }`, but I am not
touching that in this pull pending further decisions with respect to
direction.