add TransparencyLevel property to TopLevel

src/Avalonia.Controls/Platform/ITopLevelImpl.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using Avalonia.Controls;
 using Avalonia.Input;
 using Avalonia.Input.Raw;
 using Avalonia.Rendering;
@@ -106,5 +107,10 @@ namespace Avalonia.Platform
         IMouseDevice MouseDevice { get; }
 
         IPopupImpl CreatePopup();
+
+        /// <summary>
+        /// Gets or sets the <see cref="WindowTransparencyLevel"/> ofthe TopLevel.
+        /// </summary>
+        WindowTransparencyLevel TransparencyLevel { get; set; }
     }
 }

src/Avalonia.Controls/TopLevel.cs

@@ -43,6 +43,12 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<IInputElement> PointerOverElementProperty =
             AvaloniaProperty.Register<TopLevel, IInputElement>(nameof(IInputRoot.PointerOverElement));
 
+        /// <summary>
+        /// Represents the current <see cref="WindowTransparencyLevel"/>.
+        /// </summary>
+        public static readonly StyledProperty<WindowTransparencyLevel> TransparencyLevelProperty =
+            AvaloniaProperty.Register<TopLevel, WindowTransparencyLevel>(nameof(TransparencyLevel), WindowTransparencyLevel.None);
+
         private readonly IInputManager _inputManager;
         private readonly IAccessKeyHandler _accessKeyHandler;
         private readonly IKeyboardNavigationHandler _keyboardNavigationHandler;
@@ -57,6 +63,9 @@ namespace Avalonia.Controls
         static TopLevel()
         {
             AffectsMeasure<TopLevel>(ClientSizeProperty);
+
+            TransparencyLevelProperty.Changed.AddClassHandler<TopLevel>(
+                (tl, e) => { if (tl.PlatformImpl != null) tl.PlatformImpl.TransparencyLevel = (WindowTransparencyLevel)e.NewValue; });
         }
 
         /// <summary>
@@ -155,6 +164,15 @@ namespace Avalonia.Controls
             protected set { SetAndRaise(ClientSizeProperty, ref _clientSize, value); }
         }
 
+        /// <summary>
+        /// Gets or sets the <see cref="WindowTransparencyLevel"/>.
+        /// </summary>
+        public WindowTransparencyLevel TransparencyLevel
+        {
+            get { return GetValue(TransparencyLevelProperty); }
+            set { SetValue(TransparencyLevelProperty, value); }
+        }
+
         public ILayoutManager LayoutManager
         {
             get