LibHTML: Add a simple TreeNode<T> template for making trees.

We'll be making a lot of trees here, so let's share code during bootstrap.
Eventually some of these classes are gonna want custom trees but for now
we can just fit them all into the same clothes.

LibHTML/TreeNode.h

@@ -0,0 +1,61 @@
+#pragma once
+
+#include <AK/Assertions.h>
+#include <AK/NonnullRefPtr.h>
+
+template<typename T>
+class TreeNode {
+public:
+    void ref()
+    {
+        ASSERT(m_ref_count);
+        ++m_ref_count;
+    }
+
+    void deref()
+    {
+        ASSERT(m_ref_count);
+        if (!--m_ref_count)
+            delete static_cast<T*>(this);
+    }
+    int ref_count() const { return m_ref_count; }
+
+    T* parent() { return m_parent; }
+    const T* parent() const { return m_parent; }
+
+    bool has_children() const { return m_first_child; }
+    T* next_sibling() { return m_next_sibling; }
+    T* previous_sibling() { return m_previous_sibling; }
+    T* first_child() { return m_first_child; }
+    T* last_child() { return m_last_child; }
+    const T* next_sibling() const { return m_next_sibling; }
+    const T* previous_sibling() const { return m_previous_sibling; }
+    const T* first_child() const { return m_first_child; }
+    const T* last_child() const { return m_last_child; }
+
+    void append_child(NonnullRefPtr<T> node);
+
+protected:
+    TreeNode() { }
+
+private:
+    int m_ref_count { 1 };
+    T* m_parent { nullptr };
+    T* m_first_child { nullptr };
+    T* m_last_child { nullptr };
+    T* m_next_sibling { nullptr };
+    T* m_previous_sibling { nullptr };
+};
+
+template<typename T>
+inline void TreeNode<T>::append_child(NonnullRefPtr<T> node)
+{
+    ASSERT(!node->m_parent);
+    if (m_last_child)
+        m_last_child->m_next_sibling = node.ptr();
+    node->m_previous_sibling = m_last_child;
+    node->m_parent = static_cast<T*>(this);
+    m_last_child = &node.leak_ref();
+    if (!m_first_child)
+        m_first_child = m_last_child;
+}