Add recommended version to Qalculate backend

Close T3862

src/backends/qalculate/qalculatebackend.cpp

@@ -52,6 +52,11 @@ QString QalculateBackend::id() const
     return QLatin1String("qalculate");
 }
 
+QString QalculateBackend::version() const
+{
+    return QLatin1String("0.9");
+}
+
 Cantor::Session* QalculateBackend::createSession()
 {
     return new QalculateSession(this);

src/backends/qalculate/qalculatebackend.h

@@ -31,6 +31,7 @@ public:
     ~QalculateBackend();
 
     QString id() const;
+    QString version() const override;
 
     virtual Cantor::Session *createSession();
     virtual Cantor::Backend::Capabilities capabilities() const;