如何实现焦点位于QLineEdit中时，敲回车时发出一个SIGNAL
------解决方案--------------------
QLineEdit自带一个signal，可以响应回车
void QLineEdit::returnPressed () [signal]
This signal is emitted when the Return or Enter key is pressed. Note that if there is a validator() or inputMask() set on the line edit, the returnPressed() signal will only be emitted if the input follows the inputMask() and the validator() returns QValidator::Acceptable.