Add methods for easy adding/removing children

Summary:
Adds two methods ```AddChild``` and ```RemoveChild``` to be able to add/remove children without the need to know there position.
Closes https://github.com/facebook/yoga/pull/513

Reviewed By: emilsjolander

Differential Revision: D4914242

Pulled By: splhack

fbshipit-source-id: df187ba71a1bae0b3b0649624f3c29401d5f8804

csharp/Facebook.Yoga/YogaNode.cs

@@ -576,6 +576,20 @@ namespace Facebook.Yoga
             Native.YGNodeRemoveChild(_ygNode, child._ygNode);
         }
 
+        public void AddChild(YogaNode child)
+        {
+            Insert(Count, child);
+        }
+
+        public void RemoveChild(YogaNode child)
+        {
+            int index = IndexOf(child);
+            if (index >= 0)
+            {
+                RemoveAt(IndexOf(child));
+            }
+        }
+
         public void Clear()
         {
             if (_children != null)