Add settings object API

Documentation is in `docs/specs/Settings-Object-API.md`.

src/settings/settings_object.h

@@ -0,0 +1,27 @@
+#pragma once
+#include "internal/settings_object_data.h"
+
+namespace settings
+{
+class ISettingsObject
+{
+public:
+    virtual ~ISettingsObject() {}
+
+    virtual SettingsObjectData saveSettings() const = 0;
+    virtual void loadSettings( SettingsObjectData& obj ) = 0;
+
+    virtual std::string settingsName() const = 0;
+};
+
+void saveObject( const ISettingsObject& obj );
+
+void loadObject( ISettingsObject& obj );
+
+void saveNumberedObject( const ISettingsObject& obj, const int slot );
+
+void loadNumberedObject( ISettingsObject& obj, const int slot );
+
+int getAmountOfSavedObjects( ISettingsObject& obj );
+
+} // namespace settings