This changes the portlabel to an entry where you can specify the port you want to use. There is error-handling for setting a wrong value (non-int or int that's out of range).
When a port is given that is already in use, it will set an unused port. This was behaviour that was already present and I have no idea how to change it to be more consistent to the other error-handlings (i'm pretty new to all this). I hope this is ok, if not, tips to how I can make it better are more than welcome.

I don't know how to build Rhythmbox from source, so I changed the plugin in my Rhythmbox 3.4.4 installation on Kubuntu 20.04.2 and added the relevant changes here.