Libraries: Create top level directory for libraries.

Things were getting a little crowded in the project root, so this patch
moves the Lib*/ directories into Libraries/.

Libraries/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;
+}