code: def onLeftClickLabel(self, event):
          
          if event.Col == -1 and event.Row == -1:
              pass
          elif event.Col < 0:
              self.onSelectRow(event)
          elif event.Row < 0:
              self.drop_down_menu.on_label_click(event)
docs: When user clicks on a grid label, determine if it is a row label or a col label.
          Pass along the event to the appropriate function.
          (It will either highlight a column for editing all values, or highlight a row for deletion).