hw/xwin: Disable minimize button on window with skip-taskbar state

If a window has had its taskbar button removed, disable its minimize
button to prevent it becoming lost

Signed-off-by: Colin Harrison <colin.harrison@virgin.net>
Reviewed-by: Jon TURNEY <jon.turney@dronecode.org.uk>

hw/xwin/winmultiwindowwm.c

@@ -1731,6 +1731,9 @@ winApplyHints(Display * pDisplay, Window iWindow, HWND hWnd, HWND * zstyle)
     if (hint & HINT_NOSYSMENU)
         style = style & ~WS_SYSMENU;
 
+    if (hint & HINT_SKIPTASKBAR)
+        style = style & ~WS_MINIMIZEBOX;        /* window will become lost if minimized */
+
     SetWindowLongPtr(hWnd, GWL_STYLE, style);
 
     exStyle = GetWindowLongPtr(hWnd, GWL_EXSTYLE);