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.
`TestSceneStatisticsPanel` intends to check the operation of statistics
panels using dummy ruleset classes. However, `StatisticsPanel` relies on
being able to retrieve the playable beatmap, which requires a converter.
One was not provided by the dummy rulesets, therefore the retrieval
would fail with an unobserved exception.
To fix, add a barebones converter implementation that is enough for the
test to pass.
The main goal here is to remove the inheritance, since realm doesn't
like that. Unfortunate that we can't use object initialisers in a few of
these places, but no real way around that.
