Add settings API and specification

The details of the API are in the `/docs/specsSettings-API.md` file.

The enums do not list all available settings, more are to be added as
the settings API replaces inline QSettings calls.

src/utils/setup.h

@@ -17,6 +17,7 @@ enum ReturnErrorCode
     SUCCESS = 0,
     GENERAL_FAILURE = -1,
     OPENVR_INIT_ERROR = -2,
+    SETTING_INCORRECT_INDEX = -3,
 };
 
 struct CommandLineOptions