Code: Introduce pClient->setChannelName() and make GUI and JSON-RPC use it

*(Initially raised by @pljones and @ann0see in https://github.com/jamulussoftware/jamulus/pull/1975/files#r808416974)*

**Has this feature been discussed and generally agreed?**

Yes.

**Describe the solution you'd like**

https://github.com/jamulussoftware/jamulus/blob/master/src/clientsettingsdlg.cpp#L1082-L1098 + clientrpc.cpp should be refactored to use a new `pClient->setChannelName` method.

**Describe alternatives that have been considered**

Live with the duplication.

Start with the referenced lines in src/clientsettingsdlg.cpp and the related code in clientrpc.cpp, then compare how each currently handles the channel name. Refactor both paths to use the new pClient->setChannelName method; done means the GUI and JSON-RPC no longer duplicate that logic while retaining their existing behavior.

Written by the indexing model from the issue text.