text: Title: BuiltIn: New `Reset Log Level` keyword for resetting the log level to the original value
  Body: _Opened this request after the discussion under https://github.com/robotframework/robotframework/issues/4919_
  
  I'd like to request adding a new keyword `Reset Log Level` that would always reset the log level to the one set with `--loglevel` argument to robot, or to `INFO` if not set.
  
  It would ease the work as now if I want to revert to the previous log level after a `Set Log Level` keyword, I need to store the previous keyword in an extra variable.