Fix the right click menu on the column headers

[gaurav]

TableManager.defaultRightClick(MouseEvent e, int col, int row) is supposed to only display the "Row: "/"Delete this row"/"Unset this row as the reference taxon" menu items on the right click popup if you right click on a row. However, it currently displays these menu items when you right-click on the column headers as well. This suggests that the table API has changed in some way, or that there is some other bug in the code.