Change the order of the arguments on the (internal) keys_changed callback in
GSettingsListenerVTable.

This means that all functions in the table now fit the following signature:

  void (* f) (GObject             *target,
              GSettingsBackend    *backend,
              const gchar         *name_or_path,
              gpointer             origin_tag,
              const gchar * const *names);

allowing the possibility of arguments ignored at the end.

This allows us to simplify our dispatch-to-thread code in GSettingsBackend,
making it a bit less generic.

So far, this should be a straight refactor.

https://bugzilla.gnome.org/show_bug.cgi?id=710367