Backup setting on exit to plugin config dir

Only one version of the settings file will be kept per scene collection
and plugin version combination

lib/utils/plugin-state-helpers.hpp

@@ -5,6 +5,7 @@
 
 namespace advss {
 
+void SavePluginSettings(obs_data_t *);
 EXPORT void LoadPluginSettings(obs_data_t *);
 
 EXPORT void AddSaveStep(std::function<void(obs_data_t *)>);