Revert "remove portable.xaml"

This reverts commit 25d168f8cf.

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

@@ -0,0 +1,12 @@
+using System.Collections.Generic;
+using System.Runtime.Serialization;
+
+namespace Avalonia.Markup.Xaml.PortableXaml
+{
+    [DataContract]
+    class AvaloniaResourceXamlInfo
+    {
+        [DataMember]
+        public Dictionary<string, string> ClassToResourcePathIndex { get; set; } = new Dictionary<string, string>();
+    }
+}

src/Markup/Avalonia.Markup.Xaml/PortableXaml/portable.xaml.github

@@ -0,0 +1 @@
+Subproject commit 8abbe09592668efb573ac4d5548ba2d7e464ba78