added _windowProperties override (#14649)

src/Windows/Avalonia.Win32/EmbeddedWindowImpl.cs

@@ -1,13 +1,21 @@
 using System;
-using System.ComponentModel;
-using System.Runtime.InteropServices;
-using Avalonia.Platform;
+using Avalonia.Controls;
 using Avalonia.Win32.Interop;
 
 namespace Avalonia.Win32
 {
     class EmbeddedWindowImpl : WindowImpl
     {
+        public EmbeddedWindowImpl()
+        {
+            _windowProperties = new WindowProperties
+            {
+                ShowInTaskbar = false,
+                IsResizable = false,
+                Decorations = SystemDecorations.None
+            };
+        }
+
         protected override IntPtr CreateWindowOverride(ushort atom)
         {
             var hWnd = UnmanagedMethods.CreateWindowEx(