Merge branch 'add_back_postfix' into 'master'

Add BACKUP postfix to the old file

See merge request !11

app/src/updaters/appupdater.cpp

+#include <QFile>
 #include <QDebug>
 #include <QtConcurrent/QtConcurrent>
 
@@ -115,6 +116,7 @@ void AppUpdater::checkUpdateProgress()
     case Updater::STOPPING:
         break;
     case Updater::SUCCESS:
+        QFile::rename(appImagePath, appImagePath + ".BACKUP");
         updateDialog.showCompletionMessage();
         progressCheckTimer.stop();
         break;