在任務欄上顯示/隱藏窗口
WS_EX_APPWINDOW Forces a top-level window onto the taskbar when the window is visible.
這個屬性在對話框的屬性頁中是找不到的，可以直接在.rc中編輯。
另外，WS_EX_TOOLWINDOW也可以在任務欄上隱藏窗口，但是那個窗口的標題欄會使用小字體。
WS_EX_TOOLWINDOW Creates 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