Add reverse support for canonical instances

UIX/Microsoft/Iris/Layouts/DockLayoutInput.cs

@@ -9,7 +9,7 @@ using Microsoft.Iris.Library;
 
 namespace Microsoft.Iris.Layouts
 {
-    internal class DockLayoutInput : ILayoutInput, IStringEncodable
+    internal class DockLayoutInput : ILayoutInput, IStringEncodable, IHasCanonicalInstances
     {
         public static readonly DockLayoutInput Left = new DockLayoutInput();
         public static readonly DockLayoutInput Top = new DockLayoutInput();
@@ -35,12 +35,17 @@ namespace Microsoft.Iris.Layouts
                     return "Top";
                 if (this == Right)
                     return "Right";
-                return this == Bottom ? "Bottom" : "Client";
+                if (this == Bottom)
+                    return "Bottom";
+                 
+                return "Client";
             }
         }
 
         public override string ToString() => InvariantString.Format("{0}(Position={1})", GetType().Name, PositionString);
 
         public string EncodeString() => PositionString;
+
+        public string GetCanonicalName() => PositionString;
     }
 }

UIX/Microsoft/Iris/Layouts/PlacementMode.cs

@@ -6,7 +6,7 @@
 
 namespace Microsoft.Iris.Layouts
 {
-    internal class PlacementMode
+    internal class PlacementMode : IHasCanonicalInstances
     {
         private PopupPosition[] _popupPositions;
         private MouseTarget _mouseTarget;
@@ -50,7 +50,9 @@ namespace Microsoft.Iris.Layouts
 
         internal bool UsesTargetSize => _usesTargetSize;
 
-        public override string ToString()
+        public override string ToString() => GetCanonicalName() ?? base.ToString();
+
+        public string GetCanonicalName()
         {
             if (this == s_origin)
                 return "Origin";
@@ -70,7 +72,10 @@ namespace Microsoft.Iris.Layouts
                 return "MouseBottom";
             if (this == s_followMouseOrigin)
                 return "FollowMouseOrigin";
-            return this == s_followMouseBottom ? "FollowMouseBottom" : base.ToString();
+            if (this == s_followMouseBottom)
+                return "FollowMouseBottom";
+
+            return null;
         }
 
         public static PlacementMode Origin