LibHTML: Create some subdirectories.

LibHTML/DOM/ParentNode.h

@@ -0,0 +1,40 @@
+#pragma once
+
+#include <LibHTML/DOM/Node.h>
+
+class ParentNode : public Node {
+public:
+    void append_child(Retained<Node>);
+
+    Node* first_child() { return m_first_child; }
+    Node* last_child() { return m_last_child; }
+    const Node* first_child() const { return m_first_child; }
+    const Node* last_child() const { return m_last_child; }
+
+    template<typename F> void for_each_child(F) const;
+    template<typename F> void for_each_child(F);
+
+protected:
+    explicit ParentNode(NodeType type)
+        : Node(type)
+    {
+    }
+
+private:
+    Node* m_first_child { nullptr };
+    Node* m_last_child { nullptr };
+};
+
+template<typename Callback>
+inline void ParentNode::for_each_child(Callback callback) const
+{
+    for (auto* node = first_child(); node; node = node->next_sibling())
+        callback(*node);
+}
+
+template<typename Callback>
+inline void ParentNode::for_each_child(Callback callback)
+{
+    for (auto* node = first_child(); node; node = node->next_sibling())
+        callback(*node);
+}