Allow a root being a child of a root or container

Author: fubar-coder

src/FubarDev.LayoutEngine/LayoutBuilder/LayoutBuilderContainer.cs

@@ -187,6 +187,24 @@ public LayoutBuilderContainer Height(AttachedSize height)
         return this;
     }
 
+    public static LayoutBuilderContainer operator <<(LayoutBuilderContainer container, LayoutBuilderRoot item)
+    {
+        if (container._container != null)
+        {
+            throw new InvalidOperationException("Cannot modify after building.");
+        }
+
+        var oldFactory = container._containerFactory;
+        container._containerFactory = overlapLookup =>
+        {
+            var newContainer = oldFactory(overlapLookup);
+            newContainer.Add(item.Build());
+            return newContainer;
+        };
+
+        return container;
+    }
+
     public static LayoutBuilderContainer operator <<(LayoutBuilderContainer container, LayoutBuilderContainer item)
     {
         if (container._container != null)

src/FubarDev.LayoutEngine/LayoutBuilder/LayoutBuilderRoot.cs

@@ -107,6 +107,12 @@ public LayoutBuilderRoot AddOverlap(string overlappedIdentifier, LayoutBuilderIt
         return this;
     }
 
+    public static LayoutBuilderRoot operator <<(LayoutBuilderRoot root, LayoutBuilderRoot item)
+    {
+        root._root.Add(item.Build());
+        return root;
+    }
+
     public static LayoutBuilderRoot operator <<(LayoutBuilderRoot root, LayoutBuilderContainer item)
     {
         root._root.Add(item.Build(root._root));