在任务栏上显示/隐藏窗口
WS_EX_APPWINDOW Forces a top-level window onto the taskbar when the window is visible.
这个属性在对话框的属性页中是找不到的，可以直接在.rc中编辑。
另外，WS_EX_TOOLWINDOW也可以在任务栏上隐藏窗口，但是那个窗口的标题栏会使用小字体。
WS_EX_TOOLWINDOWCreates a tool window, which is a window intended to be used as a floating toolbar. A tool window has a title bar that is shorter than a normal title bar, and the window title is drawn using a smaller font. A tool window does not appear in the task bar or in the window that appears when the user presses ALT+TAB
