为什么添加的控件显示不出或者堆成了一堆， 布局了也显示不出？
Answer:从QT Assistant 中找到这句话：
Note: Creating a main window without a central widget is not supported. You must have a central widget even if it is just a placeholder.
所以要先 new 一个widget然后调用:
void setCentralWidget( QWidget* widget)
例如：我们在mainwindow中添加2个控件：QLabel 和QLineEdit:
cenWidget = new QWidget(this); //this is point to QMainWindow
setCentralWidget(cenWidget);
QHBoxLayout* H = new QHBoxLayout(cenWidget);
label = new QLabel(cenWidget);
lineEdit = new QLineEdit(cenWidget);
H->addWidget(label);
H->addWidget(label);
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