看qt的帮助文件，里面说了一句：
On X11, it is also possible to draw on the desktop.
好像说可以直接在桌面上绘制？不知道哪位高手知道怎么操作，能否给个例子，谢谢
QDesktopWidget * QApplication::desktop () [static]
Returns the desktop widget (also called the root window).
The desktop may be composed of multiple screens, so it would be incorrect, for example, to attempt to center some widget in the desktop's geometry. QDesktopWidget has various functions for obtaining useful geometries upon the desktop, such as QDesktopWidget::screenGeometry() and QDesktopWidget::availableGeometry().
On X11, it is also possible to draw on the desktop.
