Fix version checking (#4329)

https://github.com/Neargye/semver

* Use semver library for version downgrade checking

* Add test validating our current version is valid semver

src/singletons/Updates.hpp

@@ -24,6 +24,8 @@ public:
     // fourtf: don't add this class to the application class
     static Updates &instance();
 
+    static bool isDowngradeOf(const QString &online, const QString &current);
+
     void checkForUpdates();
     const QString &getCurrentVersion() const;
     const QString &getOnlineVersion() const;