Add recommended version to Maxima backend

Close T3863

src/backends/maxima/maximabackend.cpp

@@ -55,6 +55,11 @@ QString MaximaBackend::id() const
     return QLatin1String("maxima");
 }
 
+QString MaximaBackend::version() const
+{
+    return QLatin1String("5.38 and 5.39");
+}
+
 Cantor::Session* MaximaBackend::createSession()
 {
     qDebug()<<"Spawning a new Maxima session";

src/backends/maxima/maximabackend.h

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