Add access keys for menu items.

samples/ControlCatalog/MainWindow.xaml

@@ -63,11 +63,11 @@
     <Panel Margin="{Binding #MainWindow.OffScreenMargin}">
       <DockPanel LastChildFill="True" Margin="{Binding #MainWindow.WindowDecorationMargin}">
         <Menu Name="MainMenu" DockPanel.Dock="Top">
-          <MenuItem Header="File">
-            <MenuItem Header="Exit" Command="{Binding ExitCommand}" />
+          <MenuItem Header="_File">
+            <MenuItem Header="E_xit" Command="{Binding ExitCommand}" />
           </MenuItem>
-          <MenuItem Header="Help">
-            <MenuItem Header="About" Command="{Binding AboutCommand}" />
+          <MenuItem Header="_Help">
+            <MenuItem Header="_About" Command="{Binding AboutCommand}" />
           </MenuItem>
         </Menu>
         <local:MainView />