Add settings callback field to re-order setting options in GUI

src/settings_internal.h

@@ -89,6 +89,7 @@ enum SettingType {
 
 typedef bool OnChange(int32 var);           ///< callback prototype on data modification
 typedef size_t OnConvert(const char *value); ///< callback prototype for conversion error
+typedef int OnGuiOrder(uint nth);            ///< callback prototype for GUI ordering
 
 /** Properties of config file settings. */
 struct SettingDescBase {
@@ -113,6 +114,7 @@ struct SettingDesc {
 	SaveLoad save;          ///< Internal structure (going to savegame, parts to config)
 	const char *patx_name;  ///< Name to save/load setting from in PATX chunk, if NULL save/load from PATS chunk as normal
 	const char *xref;       ///< Name of SettingDesc to use instead of the contents of this one, useful for loading legacy savegames, if NULL save/load as normal
+	OnGuiOrder *orderproc;  ///< Callback procedure for GUI re-ordering
 
 	bool IsEditable(bool do_command = false) const;
 	SettingType GetType() const;