Automatic secondary sort fields

[stevesims]

It is already possible to have a secondary sort field (i.e. where one secondarily sorts based on a different column when values in the primary sort are otherwise the same) by writing a custom sort method.

An idea however could be to allow for a secondary flag on a field definition. This flag could indicate that the field should be used for secondary sorting.

Further, this could potentially allow for multiple secondary sort fields. The priority order would be the order in which fields are defined.

This may require changing how the sort methods operate - although that may be transparent to the component user.