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/backup.hpp

@@ -0,0 +1,9 @@
+#pragma once
+#include <QString>
+
+namespace advss {
+
+void AskForBackup(const QString &json);
+void BackupSettingsOfCurrentVersion();
+
+} // namespace advss