make the trayicon menu property an explicit member of trayicon.

samples/ControlCatalog/App.xaml

@@ -30,7 +30,7 @@
   <TrayIcon.Icons>
     <TrayIcons>
       <TrayIcon Icon="/Assets/test_icon.ico" ToolTipText="Avalonia Tray Icon ToolTip">
-        <NativeMenu.Menu>
+        <TrayIcon.Menu>
           <NativeMenu>
             <NativeMenuItem Header="Settings">
               <NativeMenu>
@@ -43,7 +43,7 @@
             </NativeMenuItem>
             <NativeMenuItem Header="Exit" Command="{Binding ExitCommand}" />
           </NativeMenu>
-        </NativeMenu.Menu>
+        </TrayIcon.Menu>
       </TrayIcon>
     </TrayIcons>
   </TrayIcon.Icons>

src/Avalonia.Controls/Platform/ITopLevelNativeMenuExporter.cs

@@ -7,7 +7,7 @@ namespace Avalonia.Controls.Platform
 {
     public interface INativeMenuExporter
     {
-        void SetNativeMenu(NativeMenu menu);
+        void SetNativeMenu(NativeMenu? menu);
     }
 
     public interface ITopLevelNativeMenuExporter : INativeMenuExporter

src/Avalonia.Controls/TrayIcon.cs

@@ -68,6 +68,12 @@ namespace Avalonia.Controls
         public static readonly AttachedProperty<TrayIcons> IconsProperty
             = AvaloniaProperty.RegisterAttached<TrayIcon, Application, TrayIcons>("Icons");
 
+        /// <summary>
+        /// Defines the <see cref="Menu"/> property.
+        /// </summary>
+        public static readonly StyledProperty<NativeMenu?> MenuProperty
+            = AvaloniaProperty.Register<TrayIcon, NativeMenu?>(nameof(Menu));
+
         /// <summary>
         /// Defines the <see cref="Icon"/> property.
         /// </summary>
@@ -90,6 +96,15 @@ namespace Avalonia.Controls
 
         public static TrayIcons GetIcons(AvaloniaObject o) => o.GetValue(IconsProperty);
 
+        /// <summary>
+        /// Gets or sets the Menu of the TrayIcon.
+        /// </summary>
+        public NativeMenu? Menu
+        {
+            get => GetValue(MenuProperty);
+            set => SetValue(MenuProperty, value);
+        }
+
         /// <summary>
         /// Gets or sets the icon of the TrayIcon.
         /// </summary>
@@ -155,6 +170,10 @@ namespace Avalonia.Controls
             {
                 _impl.SetToolTipText(change.NewValue.GetValueOrDefault<string?>());
             }
+            else if (change.Property == MenuProperty)
+            {
+                _impl.MenuExporter?.SetNativeMenu(change.NewValue.GetValueOrDefault<NativeMenu>());
+            }
         }
 
         /// <summary>