Compare a pair of items from the group to determine their relative sorting order. This should return 0 if the two items are at the same sorting order, a positive integer if the first item sorts after the second item, or a negative integer if the first item sorts before the second item.

Note that we don't care about the return value beyond whether it's positive, negative, or zero. This makes it especially easy to implement this method if the sorting order is determined by a property on each object that has an integer value: in this case you simply return the difference of the two property values, as in a.prop - b.prop. This will have the effect of sorting the objects in ascending order of their 'prop' property values. To sort in descending order of the same property, simply reverse the subtraction: b.prop - a.prop.

When no implementation of this method is defined in the group object, sortListGroup won't bother sorting the list at all.

By default, we don't implement this method. Subclasses that want to impose a sorting order must implement the method.

Show the group list

Sort the group list. By default, if we have a compareGroupItems() method defined, we'll sort the list using that method; otherwise, we'll just return the list unchanged.