The type of the introspect function in #GDBusSubtreeVTable.
Subtrees are flat. node, if non-
nil, is always exactly one
segment of the object path (ie: it never contains a slash).
This function should return
nil to indicate that there is no object
at this node.
If this function returns non-
nil, the return value is expected to
nil-terminated array of pointers to #GDBusInterfaceInfo
structures describing the interfaces implemented by node. This
array will have g_dbus_interface_info_unref() called on each item
before being freed with g_free().
The difference between returning
nil and an array containing zero
items is that the standard DBus interfaces will returned to the
remote introspector in the empty array case, but not in the
Gio::DBusConnection, String, String, String -> Enumerable(Gio::DBusInterfaceInfo)