Use G_ADD_PRIVATE() instead of g_type_class_add_private()

This fixes deprecation warnings about g_type_class_add_private()

The new G_ADD_PRIVATE() method has been around since GLib 2.38 (we already depend on 2.40 at minimum).

See this blog post for more details: https://www.bassi.io/articles/2013/06/21/the-king-is-dead/

Fixes tracker#115 (closed)