Consolidate Shutdown methods #1069
Description
In a recent PR (#1066), the shutdown logic for the GUIServer (bec_widgets/cli/server.py) was adapted. However, this class is only started if the gui starts through the BECIPythonClient.
Otherwise, relevant processes like the RPCServer are started through the BECDispatcher (https://github.com/bec-project/bec_widgets/blob/pre_release_v3/bec_widgets/utils/bec_dispatcher.py#L241). This is for example the case if we start the main application (bec_widgets/appplications/main_app.py).
If we start the main_app, we are now missing some of the cleanup that is implemented in the shutdown method of the GUIServer, like rpc_server.shutdown(). This should be carefully reviewed and consolidated.