Add a checkboxes settings type

Currently the checkbox settings type actually renders a checkboxes forms api element. This was a bit confusing at first since the checkbox forms api element renders a bit differently than a single option checkboxes element. Might make sense to add a checkboxes settings type and reference it in the documentation instead of checkbox? We'd need to keep the checkbox type intact to avoid BC breakage but it could just extend the checkboxes plugin with no changes. Patch incoming!