Add a warningOccurred signal for warnings that come from the backend so that the application can react to them accordingly without having to parse them via the message handler

Add a warningOccurred signal for warnings that come from the backend so that the application can react to them accordingly without having to parse them via the message handler. Since the backend is outputting via qWarning() a number of warnings which an application might find relevant then emitting a signal with these instead would be useful.