goption: Add boxed type for GOptionGroup

This would allow bindings to use _get_option_group() functions, which
would then allow them to use GOption parsing.

This also adds introspection annotations to
g_option_context_add_group(), g_option_context_set_main_group() and
g_option_context_get_main_group().

https://bugzilla.gnome.org/show_bug.cgi?id=743349