Merge pull request #1237 from jkoritzinsky/fixes/1234

Mark properties tracked by all of our XAML property types as not unknown.

src/Markup/Avalonia.Markup.Xaml/PortableXaml/AvaloniaXamlType.cs

@@ -135,6 +135,8 @@ namespace Avalonia.Markup.Xaml.PortableXaml
             };
         }
 
+        protected override bool LookupIsUnknown() => false;
+
         protected override XamlType LookupType()
         {
             var propType = GetPropertyType();
@@ -294,8 +296,6 @@ namespace Avalonia.Markup.Xaml.PortableXaml
             return Property.IsReadOnly;
         }
 
-        protected override bool LookupIsUnknown() => false;
-
         protected override Type GetPropertyType()
         {
             return Property.PropertyType;