/**
* Adds a new header to the group.
*/
public void add(final SortableTableHeader header) {
this.headers.add(header);
header.addSortToggleListener(new SortToggleListener() {
public void onSortToggle(boolean ascending) {
// Turn off the sorting on all the other headers.
int length = headers.size();
for (int i = 0; i < length; ++i) {
SortableTableHeader peer = headers.get(i);