Add orderByToggle to ListViewModel

[ascott18]

function orderByToggle(list: ListViewModel, field: string) {
  const params = list.$params;

  if (params.orderBy == field && !params.orderByDescending) {
    params.orderBy = null;
    params.orderByDescending = field;
  } else if (!params.orderBy && params.orderByDescending == field) {
    params.orderBy = null;
    params.orderByDescending = null;
  } else {
    params.orderBy = field;
    params.orderByDescending = null;
  }
}