add API to set QLoggingCategory for QJSEngine ConsoleExtension

Using the console API from JavaScript can only output to the default logging category `js`. I would like to be able to pass a QLoggingCategory to a QJSEngine to change that. The use case in Mixxx is that each hardware DJ controller is handled by one QJSEngine. Debugging messages from C++ for each controller can be given a logging category for the specific device. However debugging messages from JavaScript all go into the same logging category even if multiple devices are in use.