Implements the typical filter, where we don't care about the input data and the filter itself implements the GUI. Note when the filter property would change, the implementation must fire off a property change.
return the filter specified by the GUI. The filter string will start with the pipe character. When this would change, implementations should fire off a property change event like so:
where ff is the new value.firePropertyChange( PROP_FILTER, null, ff );
configure the GUI based on this filter. The filter string will start with the pipe character.