expose the functionality of xdg_mime_list_mime_parents

[Tomeu Vizoso <tomeu sugarlabs org>]

Hi,

what do people think about exposing that functionality as perhaps
g_content_type_get_supertypes()?

Rationale in this ticket:

http://bugzilla.gnome.org/show_bug.cgi?id=584388

Thanks,

Tomeu