Set current filename and return the associated editor instance.

def set_current_filename(self, filename, focus=True):
        """Set current filename and return the associated editor instance."""
        index = self.has_filename(filename)
        if index is not None:
            if focus:
                self.set_stack_index(index)
            editor = self.data[index].editor
            if focus:
                editor.setFocus()
            else:
                self.stack_history.remove_and_append(index)

            return editor