Fix G_TYPE_INSTANCE_GET_PRIVATE deprecation warnings

[notpeelz]

No description provided.

[vmihalko]

Previously, we used the G_DEFINE_TYPE (PolkitBackendActionPool, polkit_backend_action_pool, G_TYPE_OBJECT); to define the new type and with g_type_class_add_private (klass, sizeof (PolkitBackendActionPoolPrivate)); we add a private structure for this type; afterward we retrieve this private structure with DEPRECATED G_TYPE_INSTANCE_GET_PRIVATE macro.

This PR replaces the deprecated code with DEFINE_TYPE_WITH_PRIVATE macro which adds private instance data to the type (the g_type_class_add_private () call is not needed) and also automatically generates the getter function _get_instance_private () (the G_TYPE_INSTANCE_GET_PRIVATE is not needed).

[vmihalko]

LGTM!