[C++][Qt5] Added function to add a new Server. Removed unused variables

Created by: basyskom-dege

The issue #8694 (closed) pointed out that there are some unused variables and that it would be handy to be able to add new servers that are not provided in the OpenAPI file. The variables host, port, scheme and basePath have been removed since the servers are handled by the ServerConfiguration class. Furthermore one can add a new ServerConfiguration for an Operation:

[...]
    API api;
    //add a new server with port and server variables
    int newDefaultServerIndex =  api.addServerConfiguration("defaultServer","{server}:{port}","newly added default server", 
        QMap<QString, PFXServerVariable>{ 
        {"server", PFXServerVariable("No description provided","custom-petstore",
        QSet<QString>{ {"custom-petstore"},{"custom-qa-petstore"},{"custom-dev-petstore"} })}, 

        {"port", PFXServerVariable("No description provided","8080",
        QSet<QString>{ {"80"},{"8080"} })},  });

    //using the new servers
    api.setServerIndex("defaultServer",newDefaultServerIndex);
[...]

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