Insert the character at the current cursor position.

        This method also adds another undo-object to the undo-stack.

def keyPressEvent(self, keyEvent):
        """
        Insert the character at the current cursor position.

        This method also adds another undo-object to the undo-stack.
        """
        undoObj = UndoSelfInsert(self, keyEvent.text())
        self.qteUndoStack.push(undoObj)