Set background on MenuItem and TreeViewItem

Now we have mouse transparency with the deferred renderer, we need to make sure there's a transparent background to make them hittable.

src/Avalonia.Themes.Default/MenuItem.xaml

@@ -2,6 +2,7 @@
         xmlns:sys="clr-namespace:System;assembly=mscorlib">
   
   <Style Selector="MenuItem">
+    <Setter Property="Background" Value="Transparent"/>
     <Setter Property="BorderThickness" Value="1"/>
     <Setter Property="Padding" Value="6,0"/>
     <Setter Property="Template">

src/Avalonia.Themes.Default/TreeViewItem.xaml

@@ -31,7 +31,8 @@
   <Style Selector="TreeViewItem /template/ ToggleButton#expander">
     <Setter Property="Template">
       <ControlTemplate>
-        <Border Width="14"
+        <Border Background="Transparent"
+                Width="14"
                 Height="12"
                 HorizontalAlignment="Center"
                 VerticalAlignment="Center">