Revert "We don't actually need to Add child in AddChild"

This reverts commit c2cd3012ca.

src/Avalonia.Base/Styling/StyleBase.cs

@@ -72,11 +72,24 @@ namespace Avalonia.Styling
         public void Add(SetterBase setter) => Setters.Add(setter);
         public void Add(IStyle style) => Children.Add(style);
 
-        void IAddChild.AddChild(object child) { }
+        void IAddChild.AddChild(object child)
+        {
+            switch (child)
+            {
+                case SetterBase setter:
+                    Setters.Add(setter);
+                    break;
+                case IStyle style:
+                    Children.Add(style);
+                    break;
+                default:
+                    throw new InvalidOperationException($"Cannot add {child.GetType()} to a style.");
+            }
+        }
 
-        void IAddChild<SetterBase>.AddChild(SetterBase setter) { }
+        void IAddChild<SetterBase>.AddChild(SetterBase setter) => Setters.Add(setter); 
 
-        void IAddChild<IStyle>.AddChild(IStyle style) { }
+        void IAddChild<IStyle>.AddChild(IStyle style) => Children.Add(style);
 
         public event EventHandler? OwnerChanged;