设置占位符
ui->lineEdit->setPalaceholderText("QQ号/手机/邮箱");设置输入为黑点
ui->lineEdit_2->setEchoMode(QLineEdit::Password);默认为：
QLineEdit::Normal 0 Display characters as they are entered. This is the default.
不要显示任何东西。这可能适用于密码长度都应该保密的密码。
QLineEdit::NoEcho 1 Do not display anything. This may be appropriate for passwords where even the length of the password should be kept secret.
显示与平台相关的密码掩码字符，而不是实际输入的字符。
QLineEdit::Password 2 Display platform-dependent password mask characters instead of the characters actually entered.
在编辑时按输入显示字符，否则按密码显示字符。
QLineEdit::PasswordEchoOnEdit 3 Display characters as they are entered while editing otherwise display characters as with Password.
