Improve the QML LoggingCategory

Right now the QML LoggingCategory only suites the need when very simple categorized logging should be done.

Once you want to achieve the same optimizations like in C++ you run into various problems e.g.

There is no way to enable/disable the logging of certain message types from QML.
There is no way to know which messages types are enabled from QML. This would be needed to write more optimized logging and only calculate values when really needed for logging them.

If possible it would also be great if we could optimize the console.log() call in the same way as it is done in C++ to not even evaulate the arguments when the logging category passed is not enabled.