Fixed PopUpBox VerticalContentAlignment (#3864)

Author: JorisCleVR

src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.PopupBox.xaml

@@ -80,6 +80,7 @@
     <Setter Property="PopupHorizontalOffset" Value="0" />
     <Setter Property="PopupUniformCornerRadius" Value="2" />
     <Setter Property="PopupVerticalOffset" Value="0" />
+    <Setter Property="VerticalContentAlignment" Value="Center" />
     <Setter Property="Template">
       <Setter.Value>
         <ControlTemplate TargetType="{x:Type wpf:PopupBox}">