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/LibGUI/GModelIndex.h

@@ -0,0 +1,42 @@
+#pragma once
+
+class GModel;
+
+class GModelIndex {
+    friend class GModel;
+
+public:
+    GModelIndex() {}
+
+    bool is_valid() const { return m_row != -1 && m_column != -1; }
+    int row() const { return m_row; }
+    int column() const { return m_column; }
+
+    void* internal_data() const { return m_internal_data; }
+
+    GModelIndex parent() const;
+
+    bool operator==(const GModelIndex& other) const
+    {
+        return m_model == other.m_model && m_row == other.m_row && m_column == other.m_column && m_internal_data == other.m_internal_data;
+    }
+
+    bool operator!=(const GModelIndex& other) const
+    {
+        return !(*this == other);
+    }
+
+private:
+    GModelIndex(const GModel& model, int row, int column, void* internal_data)
+        : m_model(&model)
+        , m_row(row)
+        , m_column(column)
+        , m_internal_data(internal_data)
+    {
+    }
+
+    const GModel* m_model { nullptr };
+    int m_row { -1 };
+    int m_column { -1 };
+    void* m_internal_data { nullptr };
+};