make app menu declaration sensible.

samples/ControlCatalog/App.xaml

@@ -19,20 +19,14 @@
   </Application.Styles>
 
   <NativeMenu.Menu>
-    <NativeMenu>
-      <NativeMenuItem>
-        <NativeMenuItem.Menu>
-          <NativeMenu>
-            <NativeMenuItem Header="Open" Clicked="OnOpenClicked"/>
-            <NativeMenuItem Header="Recent">
-              <NativeMenuItem.Menu>
-                <NativeMenu/>
-              </NativeMenuItem.Menu>
-            </NativeMenuItem>
-            <NativeMenuItem Header="Quit Avalonia" Gesture="CMD+Q"/>
-          </NativeMenu>
-        </NativeMenuItem.Menu>
-      </NativeMenuItem>
-    </NativeMenu>
+      <NativeMenu>
+        <NativeMenuItem Header="Open" Clicked="OnOpenClicked"/>
+        <NativeMenuItem Header="Recent">
+          <NativeMenuItem.Menu>
+            <NativeMenu/>
+          </NativeMenuItem.Menu>
+        </NativeMenuItem>
+        <NativeMenuItem Header="Quit Avalonia" Gesture="CMD+Q"/>
+      </NativeMenu>
   </NativeMenu.Menu>
 </Application>

samples/ControlCatalog/App.xaml.cs

@@ -16,7 +16,7 @@ namespace ControlCatalog
 
             Name = "Avalonia";
 
-            _recentMenu = ((NativeMenu.GetMenu(this).Items[0] as NativeMenuItem).Menu.Items[1] as NativeMenuItem).Menu;
+            _recentMenu = (NativeMenu.GetMenu(this).Items[1] as NativeMenuItem).Menu;
         }
 
         public void OnOpenClicked(object sender, EventArgs args)

src/Avalonia.Native/AvaloniaNativeMenuExporter.cs

@@ -276,8 +276,17 @@ namespace Avalonia.Native
                     appMenu = _factory.CreateMenu();
                 }
 
+                var menuItem = new NativeMenuItem();
+
+                menuItem.Menu = new NativeMenu();
+
+                foreach(var item in menuItems)
+                {
+                    menuItem.Menu.Add(item);
+                }
+
                 appMenu.Clear();
-                AddItemsToMenu(appMenu, menuItems);
+                AddItemsToMenu(appMenu, new List<NativeMenuItemBase> { menuItem });
 
                 _factory.SetAppMenu(appMenu);
             }