- Don't use @parent_class name. This local variable (and @object_class) is
  the class instance up-cast to the pointer types of the parents. The point
  here is not that it is the direct parent. The point is, that it's the
  NMSettingClass type.
  Also, it can only be used inconsistently, in face of NMSettingIP4Config,
  who's parent type is NMSettingIPConfig. Clearly, inside
  nm-setting-ip4-config.c we wouldn't want to use the "parent_class"
  name. Consistently rename @parent_class to @setting_class.

- Also rename the pointer to the own class to @klass. "setting_class" is also the
  wrong name for that, because the right name would be something like
  "setting_6lowpan_class".
  However, "klass" is preferred over the latter, because we commonly create new
  GObject implementations by copying an existing one. Generic names like "klass"
  and "self" inside a type implementation make that simpler.

- drop useless comments like

     /* virtual functions */
     /* Properties */

  It's better to logically and visually structure the code, and avoid trival
  remarks about that. They only end up being used inconsistently. If you
  even need a stronger visual separator, then an 80 char /****/ line
  should be preferred.