v1.11

主窗口隐藏，子窗口如果关闭，主窗口也会被关闭。
应该是下面这个问题。把 ScreenShoot 中的clos()改为hide()解决
The QApplication::lastWindowClosed() signal is emitted when the last
visible primary window (i.e. window with no parent) with the
Qt::WA_QuitOnClose attribute set is closed. By default this attribute is
set for all widgets except transient windows such as splash screens,
tool windows, and popup menus.