Added vertical offset to properly align submenu (#2201)

Author: jizc

MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.Menu.xaml

@@ -207,6 +207,7 @@
                                IsOpen="{Binding IsSubmenuOpen, RelativeSource={RelativeSource TemplatedParent}}"
                                PopupAnimation="Slide"
                                Placement="Bottom"
+                               VerticalOffset="-16"
                                 CacheMode="{Binding RelativeSource={RelativeSource Self}, Path=(wpf:ShadowAssist.CacheMode)}">
                             <Border x:Name="SubMenuBorder"
                                     Background="{Binding Path=Background, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=MenuBase}}"