NMSetting internally already tracked a list of all proper GObject properties
and D-Bus-only properties.

Rework the tracking of the list, so that:

- instead of attaching the data to the GType of the setting via
  g_type_set_qdata(), it is tracked in a static array indexed by
  NMMetaSettingType. This allows to find the setting-data by simple
  pointer arithmetic, instead of taking a look and iterating (like
  g_type_set_qdata() does).

  Note, that this is still thread safe, because the static table entry is
  initialized in the class-init function with _nm_setting_class_commit().
  And it only accessed by following a NMSettingClass instance, thus
  the class constructor already ran (maybe not for all setting classes,
  but for the particular one that we look up).

  I think this makes initialization of the metadata simpler to
  understand.
  Previously, in a first phase each class would attach the metadata
  to the GType as setting_property_overrides_...