code: def count_channels(self):
          
          merge = self.index['merge']
  
          if len(self.idx_chan.selectedItems()) > 1:
              if merge.isEnabled():
                  return
              else:
                  merge.setEnabled(True)
          else:
              self.index['merge'].setCheckState(Qt.Unchecked)
              self.index['merge'].setEnabled(False)
docs: If more than one channel selected, activate merge checkbox.