Ability to set MyGUI logging verbosity

I haven't found an environment variable to do that, MyGUI logging isn't particularly useful for OpenMW like OpenMW's own logging, and it would be nice to have an option to disable it or an environment variable like the one we have for OpenMW logging levels.

This would particularly be useful to minimize logging performance impact on the game on Android devices (where it has a more visible impact than on PC).