Add fallback to update mechanism

This brings a fallback version check to the update mechanism,
which should be as stable as it can be.
It will allow migrating to another primary mechanism without
having to have updated the older primary mechanism too.

src/gui/programupdater.h

@@ -55,8 +55,14 @@ signals:
 
 private slots:
     void rssDownloadFinished(const Net::DownloadResult &result);
+    void fallbackDownloadFinished(const Net::DownloadResult &result);
 
 private:
-    QString m_newVersion;
+    void handleFinishedRequest();
+    bool shouldUseFallback() const;
+
+    mutable bool m_hasCompletedOneReq = false;
+    QString m_remoteVersion;
+    QString m_fallbackRemoteVersion;
     QUrl m_updateURL;
 };