继承QLineEdit 重载下面的方法textEdited
见说明
void QLineEdit::textEdited ( const QString & text ) [signal]
This signal is emitted whenever the text is edited. The text argument is the next text.
Unlike textChanged(), this signal is not emitted when the text is changed programmatically, for example, by calling setText().
当每次用户输入时,触发该事件,text即为输入
如果长度不超过界限 则调用setText设置输入 否则不输入