WIP: using macOS NativeMenu API

src/GUI/App.axaml

@@ -1,6 +1,7 @@
 <Application xmlns="https://github.com/avaloniaui"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              x:Class="GUI.App"
+             Name="Resonance"
              xmlns:local="using:GUI"
              RequestedThemeVariant="Default">
              <!-- "Default" ThemeVariant follows system theme variant. "Dark" or "Light" are other available options. -->
@@ -13,7 +14,6 @@
     <NativeMenu.Menu>
         <NativeMenu>
             <NativeMenuItem Header="关于…" />
-            <NativeMenuItemSeparator></NativeMenuItemSeparator>
             <NativeMenuItem Header="偏好设置…"/>
         </NativeMenu>
     </NativeMenu.Menu>

src/GUI/Views/MainWindow.axaml

@@ -17,6 +17,16 @@
         <vm:MainWindowViewModel/>
     </Design.DataContext>
     
+    <NativeMenu.Menu>
+        <NativeMenu>
+            <NativeMenuItem Header="Database" IsVisible="True">
+                <NativeMenu>
+                    <NativeMenuItem Header="Open database file in Finder..."/>
+                </NativeMenu>
+            </NativeMenuItem>
+        </NativeMenu>
+    </NativeMenu.Menu>
+    
     <Grid RowDefinitions="Auto,*,Auto">
 
         <!-- 工具栏 -->