change the default port of the Qt VNC Server to 5901

The default port of the Qt VNC Server is 5900. On macOS this port is used by Apple Screen Sharing. This makes it impossible to use the example and the code with defaults since any client connects to Apple Screen Sharing and to Qt VNC Server. It is also weird that the Qt VNC Chat reports running on the 5900, whereas this port is definitely busy in macOS.

Adding setting the port to 5901 in the code of the example solves the problem:

        //! [vncitem]
        VncItem {
            id: grabber
            anchors.fill: parent
            vncPort: 5901

I think the problem will occur on any other system with another VNC-based software running in the background