ArkestraGenericLister.determine_layout_settings() - issues with querysets

[evildmp]

If ArkestraGenericLister.determine_layout_settings() receives a horizontal list_ it needs to add lastcolumn to the class of the last item.

Sometimes the list will be a queryset, in which case using a negative slice would break.

Currently we coerce it to a list just in case. Maybe there is a better way of doing this.