checkUpates: installer support (Win), use GUI buildTag and version

src/main/oshelper.h

@@ -36,6 +36,8 @@
 class OSHelper : public QObject
 {
     Q_OBJECT
+    Q_PROPERTY(bool installed READ installed CONSTANT);
+
 public:
     explicit OSHelper(QObject *parent = 0);
 
@@ -47,6 +49,9 @@ public:
     Q_INVOKABLE bool removeTemporaryWallet(const QString &walletName) const;
     Q_INVOKABLE bool isCapsLock() const;
 
+private:
+    bool installed() const;
+
 signals:
 
 public slots: