code: def on_category_add_clicked(self, button):
          
  
          new_category = self.category_store.insert_before(self.category_store.unsorted_category,
                                                           [-2, _("New category")])
  
          model = self.category_tree.get_model()
  
          self.categoryCell.set_property("editable", True)
          self.category_tree.set_cursor_on_cell(model.get_path(new_category),
                                           focus_column = self.category_tree.get_column(0),
                                           focus_cell = None,
                                           start_editing = True)
docs: appends row, jumps to it and allows user to input name