Fix things mentioned during code review

Ref: https://github.com/dolphin-emu/dolphin/pull/4917

Source/Core/Common/Config/Layer.h

@@ -0,0 +1,80 @@
+// Copyright 2016 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <map>
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "Common/Config/Enums.h"
+#include "Common/Config/Section.h"
+
+namespace Config
+{
+using LayerMap = std::map<System, std::vector<Section>>;
+
+class ConfigLayerLoader
+{
+public:
+  explicit ConfigLayerLoader(LayerType layer);
+  virtual ~ConfigLayerLoader();
+  virtual void Load(Layer* config_layer) = 0;
+  virtual void Save(Layer* config_layer) = 0;
+
+  LayerType GetLayer() const;
+
+private:
+  const LayerType m_layer;
+};
+
+class Layer
+{
+public:
+  explicit Layer(LayerType layer);
+  explicit Layer(std::unique_ptr<ConfigLayerLoader> loader);
+  virtual ~Layer();
+
+  // Convenience functions
+  bool Exists(System system, const std::string& section_name, const std::string& key);
+  bool DeleteKey(System system, const std::string& section_name, const std::string& key);
+  template <typename T>
+  bool GetIfExists(System system, const std::string& section_name, const std::string& key, T* value)
+  {
+    if (Exists(system, section_name, key))
+      return GetOrCreateSection(system, section_name)->Get(key, value);
+
+    return false;
+  }
+
+  virtual Section* GetSection(System system, const std::string& section_name);
+  virtual Section* GetOrCreateSection(System system, const std::string& section_name);
+
+  // Explicit load and save of layers
+  void Load();
+  void Save();
+
+  LayerType GetLayer() const;
+  const LayerMap& GetLayerMap() const;
+  // Stay away from this routine as much as possible
+  ConfigLayerLoader* GetLoader() const;
+
+protected:
+  bool IsDirty() const;
+  void ClearDirty();
+
+  LayerMap m_sections;
+  const LayerType m_layer;
+  std::unique_ptr<ConfigLayerLoader> m_loader;
+};
+
+class RecursiveLayer final : public Layer
+{
+public:
+  RecursiveLayer();
+  Section* GetSection(System system, const std::string& section_name) override;
+  Section* GetOrCreateSection(System system, const std::string& section_name) override;
+};
+}