This allows datafilters to include additional fields other than the standard list of selected values.
For example they may wish to include an additional select or checkbox to change how the values are used in the resulting query.
The previous behaviour guessed the table filterset based on the table's
class name, but this is not very flexible if you wish to create an
abstract table class and use a generic filter for all implementations.
This change adds the ability to specify the filterset class, with the
default behaviour using the table class name as a prefix.
Mark Johnson