Move tab.

        (tabs themselves have already been moved by the history.tabwidget)

def move_tab(self, index_from, index_to):
        """
        Move tab.

        (tabs themselves have already been moved by the history.tabwidget)
        """
        filename = self.filenames.pop(index_from)
        editor = self.editors.pop(index_from)

        self.filenames.insert(index_to, filename)
        self.editors.insert(index_to, editor)