feat: logging levels

The constructor of EventSystem takes a boolean parameter for debug mode, in which every request, notification, error, etc. is written to the console. Once a plugin stack has reached sufficient complexity, it becomes almost impossible to find errors and important console output in a sea of banal trace statements as every request is printed. So, let's replace the boolean parameter with some logging levels. Preliminary suggestion: always print error and info-type (e.g. "loaded plugin xyz") messages and have debug (???) and trace (print everything, as is currently done) levels.