win32: make setting ShowInTaskbar = true on an owned window that is already shown, not remove the ownership

This makes us consistent with WPF.

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -683,9 +683,10 @@ namespace Avalonia.Win32
             if (parentHwnd == IntPtr.Zero && !_windowProperties.ShowInTaskbar)
             {
                 parentHwnd = OffscreenParentWindow.Handle;
-                _hiddenWindowIsParent = true;
             }
 
+            _hiddenWindowIsParent = parentHwnd == OffscreenParentWindow.Handle;
+
             SetWindowLongPtr(_hwnd, (int)WindowLongParam.GWL_HWNDPARENT, parentHwnd);
         }