LibHTML: Add inserted_into() and removed_from() TreeNode callbacks

These will be called when a Node or LayoutNode is inserted or removed
from a tree. They get the parent node as an argument.

Libraries/LibHTML/DOM/Node.h

@@ -55,6 +55,9 @@ public:
         return nullptr;
     }
 
+    virtual void inserted_into(Node&) {}
+    virtual void removed_from(Node&) {}
+
 protected:
     Node(Document&, NodeType);