chore(style): add Menu small style.

src/Shared/HandyControl_Shared/Themes/Styles/Menu.xaml

@@ -1,7 +1,14 @@
-<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation">
+<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
 
     <Style BasedOn="{StaticResource MenuItemBaseStyle}" TargetType="MenuItem" />
 
     <Style BasedOn="{StaticResource MenuBaseStyle}" TargetType="Menu" />
 
+    <Style x:Key="Menu.Small" BasedOn="{StaticResource MenuBaseStyle}" TargetType="Menu">
+        <Setter Property="hc:MenuAttach.ItemPadding" Value="6,0" />
+        <Setter Property="hc:MenuAttach.PopupVerticalOffset" Value="-3" />
+        <Setter Property="hc:MenuAttach.PopupHorizontalOffset" Value="6" />
+    </Style>
+
 </ResourceDictionary>